qemu: Fix GIC behavior for the default case

qemu: Fix GIC behavior for the default case

When no GIC version is specified, we currently default to GIC v2;
however, that's not a great default, since guests will fail to
start if the hardware only supports GIC v3.

Change the behavior so that a sensible default is chosen instead.
That basically means using the same algorithm whether the user
didn't explicitly enable the GIC feature or they explicitly
enabled it but didn't specify any GIC version.

Signed-off-by: Andrea Bolognani <abologna@redhat.com>
Reviewed-by: John Ferlan <jferlan@redhat.com>