Fix kernel command line from kvm-test.sh

Since 06ac3f92, we invoke kvm directly through subprocess.run.
Therefore, we must not add extra quotes around the -append options,
otherwise they persist and are passed in the kernel command line:

  $ cat /proc/cmdline
  "autoinstall subiquity-channel=stable" initrd=initrd

Later on, we fail to parse "autoinstall" and "subiquity-channel=stable"
as two distinct options.

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

scripts/kvm-test.py

@@ -431,8 +431,7 @@ def install(ctx):
                 # kernel / initrd
                 kvm.extend(('-kernel', f'{mntdir}/casper/vmlinuz'))
                 kvm.extend(('-initrd', get_initrd(mntdir)))
-                toappend = ' '.join(appends)
-                kvm.extend(('-append', f'"{toappend}"'))
+                kvm.extend(('-append', ' '.join(appends)))
                 run(kvm)
         else:
             run(kvm)