kvm-test: introduce ability to pass -nic options to QEMU

The only way we had to pass -nic options to QEMU was to specify the
number of networks using the --nets option. Depending on its value, the
option would instruct QEMU to:
 * instantiate *n* user host networks if --nets >= 1
 * instantiate no network if --nets is 0
 * instantiate a "dead" network if --nets < 0

To simulate more advanced networks (such as networks where HTTP proxy is
needed), we want to add the possibility to use TAP network interfaces.

This patch adds the ability to pass custom -nic options to QEMU.

Three new options are available:

 * The --nic option passes the argument as-is to QEMU -nic option
 * The --nic-user option passes a user host network -nic option to QEMU
   - with SSH forwarding enabled. This is just like we do when using the
     --nets >= 1 option)
 * The --net-tap takes the name of an existing tap interface and
   generates the following -nic argument:

    tap,id={ifname},ifname={ifname},script=no,downscript=no,model=e1000

In the example below:
 * the first network uses the tap0 interface.
 * the second network uses the tap1 interface (it is syntactically
   equivalent to the first one)
 * the third network uses a user host network (with SSH forwarding)

  $ kvm-test.py \
    --nic tap,id=tap0,ifname=tap0,script=no,downscript=no,model=e1000 \
    --nic-tap tap1 \
    --nic-user

Signed-off-by: Olivier Gayot <olivier.gayot@canonical.com>

scripts/kvm-test.py

@@ -19,7 +19,7 @@ import socket
 import subprocess
 import sys
 import tempfile
-from typing import Tuple
+from typing import List, Tuple
 import yaml
 
 
@@ -47,6 +47,11 @@ def salted_crypt(plaintext_password):
     return crypt.crypt(plaintext_password, salt)
 
 
+class Tap:
+    def __init__(self, ifname: str) -> None:
+        self.ifname = ifname
+
+
 class Context:
     def __init__(self, args):
         self.config = self.load_config()
@@ -149,6 +154,18 @@ parser.add_argument('-i', '--img', action='store', help='use this img')
 parser.add_argument('-n', '--nets', action='store', default=1, type=int,
                     help='''number of network interfaces.
                     0=no network, -1=deadnet''')
+parser.add_argument('--nic-user', action="append_const", dest="nics",
+                    const=None,
+                    help='pass user host -nic to QEMU'
+                         ' - overrides --nets')
+parser.add_argument('--nic-tap', action="append", dest="nics", type=Tap,
+                    metavar="ifname",
+                    help='TAP interface to be passed as -nic to QEMU'
+                         ' - overrides --nets')
+parser.add_argument('--nic', action="append", dest="nics",
+                    metavar="argument",
+                    help='pass custom -nic argument to QEMU'
+                         ' - overrides --nets')
 parser.add_argument('-o', '--overwrite', default=False, action='store_true',
                     help='allow overwrite of the target image')
 parser.add_argument('-q', '--quick', default=False, action='store_true',
@@ -338,23 +355,59 @@ class PortFinder:
         return next(self.finder)
 
 
-def nets(ctx):
-    if ctx.args.nets > 0:
-        ports = PortFinder()
-        ret = []
-        for _ in range(ctx.args.nets):
-            port = ports.get()
-            ret.extend(('-nic',
-                        'user,model=virtio-net-pci,' +
-                        f'hostfwd=tcp::{port}-:22'))
-        return ret
-    elif ctx.args.nets == 0:
-        # no network
-        return ('-nic', 'none')
-    else:
-        # nic present but restricted - simulate deadnet environment
+class NetFactory:
+    """ Generate -nic options for QEMU. """
+    ports_finder = PortFinder()
+
+    def user(self) -> Tuple[str, ...]:
+        """ User host network with SSH forwarding """
+        port = self.ports_finder.get()
+        return ('-nic', f'user,model=virtio-net-pci,hostfwd=tcp::{port}-:22')
+
+    def tap(self, ifname: str) -> Tuple[str, ...]:
+        """ Network using an existing TAP interface. """
+        tap_props = {
+            "id": ifname,
+            "ifname": ifname,
+            "script": "no",
+            "downscript": "no",
+            "model": "e1000",
+        }
+
+        nic = ",".join(["tap"] + [f"{k}={v}" for k, v in tap_props.items()])
+
+        return ('-nic', nic)
+
+    def deadnet(self) -> Tuple[str, ...]:
+        """ NIC present but restricted - simulate deadnet environment """
         return ('-nic', 'user,model=virtio-net-pci,restrict=on')
 
+    def nonet(self) -> Tuple[str, ...]:
+        """ No network """
+        return ('-nic', 'none')
+
+
+def nets(ctx) -> List[str]:
+    nics: List[str] = []
+    factory = NetFactory()
+
+    if ctx.args.nics:
+        for nic in ctx.args.nics:
+            if nic is None:
+                nics.extend(factory.user())
+            elif isinstance(nic, Tap):
+                nics.extend(factory.tap(nic.ifname))
+            else:
+                nics.extend(('-nic', nic))
+    elif ctx.args.nets > 0:
+        for _ in range(ctx.args.nets):
+            nics.extend(factory.user())
+    elif ctx.args.nets == 0:
+        nics.extend(factory.nonet())
+    else:
+        nics.extend(factory.deadnet())
+    return nics
+
 
 def bios(ctx):
     ret = []