libxl: Domain event handler improvements

libxl: Domain event handler improvements

Since libxl provides the domain ID in the event handler callback,
find the domain object based on the ID.  This approach prevents
processing the callback on a domain that has already been reaped.

Also, similar to the xl implementation, ignore the SUSPEND shutdown
reason.  By calling libxl_domain_suspend(), we know a shutdown
event with SUSPEND reason will be generated, but it can be safely
ignored since any subsequent cleanup will be done by the callers.