virtio: serial: expose a 'guest_writable' callback for users

Users of virtio-serial may want to know when a port becomes writable.  A
port can stop accepting writes if the guest port is open but not being
read from.  In this case, data gets queued up in the virtqueue, and
after the vq is full, writes to the port do not succeed.

When the guest reads off a vq element, and adds a new one for the host
to put data in, we can tell users the port is available for more writes,
via the new ->guest_writable() callback.

Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Amit Shah <amit.shah@redhat.com>
