Commit 6b331efb broke the s390 proxy version
of virtio-serial by only taking its PCI brother into account.

So let's adjust s390-virtio-serial the same way as its PCI counterpart, making
it compile and work again.
Signed-off-by: NAlexander Graf <agraf@suse.de>
CC: Amit Shah <amit.shah@redhat.com>
Signed-off-by: NAurelien Jarno <aurelien@aurel32.net>