xc_save: ignore the first suspend event channel notification

xc_save: ignore the first suspend event channel notification

I've noticed that the suspend event channel becomes pending as soon as
it is bound. I'm not sure why or whether this is intentional, but it
means that the suspend function will return before the domain has
completed suspending unless the first notification is cleared. Without
this patch, xc_domain_save may find that the guest has not suspended
and sleep in 10ms chunks until it does. Typically this is several
milliseconds of wasted time.

From: Brendan Cully <brendan@cs.ubc.ca>
Signed-off-by: Keir Fraser <keir.fraser@citrix.com>