xen/pvcalls: disconnect and module_exit

Implement backend_disconnect. Call pvcalls_back_release_active on active
sockets and pvcalls_back_release_passive on passive sockets.

Implement module_exit by calling backend_disconnect on frontend
connections.

[ boris: fixed long lines ]

Signed-off-by: Stefano Stabellini <stefano@aporeto.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
CC: boris.ostrovsky@oracle.com
CC: jgross@suse.com
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>