Add facility to get notification of domain suspend by event channel.

Add facility to get notification of domain suspend by event channel.
This event channel will be notified when the domain transitions to the
suspended state, which can be much faster than raising VIRQ_DOM_EXC
and waiting for the notification to be propagated via xenstore.

No attempt is made here to prevent multiple subscribers (last one
wins), or to detect that the subscriber has gone away. Userspace tools
should take care.

Signed-off-by: Brendan Cully <brendan@cs.ubc.ca>