xen-kbdfront: handle backend CLOSED without CLOSING

xen-kbdfront: handle backend CLOSED without CLOSING

Backend drivers shouldn't transistion to CLOSED unless the frontend is
CLOSED.  If a backend does transition to CLOSED too soon then the
frontend may not see the CLOSING state and will not properly shutdown.

So, treat an unexpected backend CLOSED state the same as CLOSING.

Acked-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Signed-off-by: David Vrabel <david.vrabel@citrix.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
(cherry picked from commit 2ebb939ab9c6a2484866c5eae4184c83c2b21af8)