virtio-console-test: Test virtserialport as well

A test is only as good as its coverage - testing virtserialport in
addition to virtconsole showed that commit
0399a381 (virtio-console: QOM cast
cleanup for VirtConsole) broke virtserialport.

Acked-by: Richard W.M. Jones <rjones@redhat.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>