xenbus: rebind irq on restore

When restoring, rebind the existing xenbus irq to the new xenbus event
channel.  (It turns out in practice that this is always the same, and
is never updated on restore.  That's a bug, but Xeno-linux has been
like this for a long time, so it can't really be fixed.)

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>