virtio: console: fix race in port_fops_open() and port unplug

Between open() being called and processed, the port can be unplugged.
Check if this happened, and bail out.

A simple test script to reproduce this is:

while true; do for i in $(seq 1 100); do echo $i > /dev/vport0p3; done; done;

This opens and closes the port a lot of times; unplugging the port while
this is happening triggers the bug.

CC: <stable@vger.kernel.org>
Signed-off-by: Amit Shah <amit.shah@redhat.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>