tests.virtio_console: Add boot_nr1 test

To make sure boot_nr tests are working correctly it's necessarily to boot
(and open) the port with nr=1.

Signed-off-by: Lukáš Doktor <ldoktor@redhat.com>
Merge pull request #315 from ldoktor/virtio_console

tests.virtio_console: Add boot_nr1 test