virtio-console: Fix VIRTIO_CONSOLE() cast macro

Commit 0399a3819b27083ba69b88a9baa9025facab85bd (virtio-console: QOM
cast cleanup for VirtConsole) broke virtserialport since it shares
functions and state struct with virtconsole. Let virtconsole inherit
from virtserialport, and use virtserialport type for casting.

Note that virtio-serial-port is the abstract base type in
virtio-serial-bus.c, whereas virtserialport is the user-instantiatable
type in virtio-console.c. Therefore using TYPE_VIRTIO_CONSOLE_SERIAL_PORT.

Reported-by: Richard W.M. Jones <rjones@redhat.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>
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
0399a3819b27083ba69b88a9baa9025facab85bd (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>