virtio: console: Ensure only one process can have a port open at a time

Add a guest_connected field that ensures only one process
can have a port open at a time.

This also ensures we don't have a race when we later add support for
dropping buffers when closing the char dev and buffer caching is turned
off for the particular port.

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