Fix -drive option from kvm-test.sh

The function drive() used to return a string in the following format:

  "-drive file=/path/to/iso,..."

However, qemu/kvm expects "-drive" to be an argument and
"file=/path/to/iso,..." to be another argument.

The command was constructed as below since the beginning:

  kvm = [
     "kvm",
     "-cdrom", "custom.iso",          # <- OK
     "-drive file=/path/to/iso,...",  # <- NOK
  ]

Before 06ac3f92, we would join all the arguments using spaces before
executing the kvm command. Therefore we would luckily end up with a
correct command:

  " ".join(kvm)  ->  "kvm -cdrom custom.iso -drive file=/path/to/iso,..."

However, now that we supply the command to subprocess.run directly, the
problem shows up.

Fixed by returning a tuple("-drive", "file=/path/to/iso,...") from
the drive() function.

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

scripts/kvm-test.py

@@ -20,6 +20,7 @@ import socket
 import subprocess
 import sys
 import tempfile
+from typing import Tuple
 import yaml
 
 
@@ -307,7 +308,8 @@ def create_seed(cloudconfig, tempdir):
     return seed
 
 
-def drive(path, format='qcow2'):
+def drive(path, format='qcow2') -> Tuple[str, str]:
+    """ Return a tuple (-drive, <options>) that can be passed to kvm """
     kwargs = []
     serial = None
     cparam = 'writethrough'
@@ -318,7 +320,7 @@ def drive(path, format='qcow2'):
     if serial:
         kwargs.append(f'serial={serial}')
 
-    return ['-drive', ','.join(kwargs)]
+    return ('-drive', ','.join(kwargs))
 
 
 class PortFinder: