virtio-serial: Use control messages to notify guest of new ports

virtio-serial: Use control messages to notify guest of new ports

Allow the port 'id's to be set by a user on the command line. This is
needed by management apps that will want a stable port numbering scheme
for hot-plug/unplug and migration.

Since the port numbers are shared with the guest (to identify ports in
control messages), we just send a control message to the guest
indicating addition of new ports (hot-plug) or notifying the guest of
the available ports when the guest sends us a DEVICE_READY control
message.

Signed-off-by: Amit Shah <[email protected]>
Signed-off-by: Anthony Liguori <[email protected]>