When trying to specify an input device on s390x without bus like this:

 <input type='keyboard'/>

... then libvirt currently complains:

 error: unsupported configuration: USB is disabled for this domain,
 but USB devices are present in the domain XML

This is somewhat confusing since the user did not specify an USB
device here. Since USB is not available on s390x, we should default
to the "virtio" bus here instead.

Buglink: https://bugzilla.redhat.com/show_bug.cgi?id=1790189Signed-off-by: NThomas Huth <thuth@redhat.com>
Reviewed-by: NMichal Privoznik <mprivozn@redhat.com>