Add DOMCTL to limit the number of event channels a domain may use

Add DOMCTL to limit the number of event channels a domain may use

Add XEN_DOMCTL_set_max_evtchn which may be used during domain creation to
set the maximum event channel port a domain may use.  This may be used to
limit the amount of Xen resources (global mapping space and xenheap) that
a domain may use for event channels.

A domain that does not have a limit set may use all the event channels
supported by the event channel ABI in use.

Signed-off-by: David Vrabel <david.vrabel@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Daniel De Graaf <dgdegra@tycho.nsa.gov>
Acked-by: Keir Fraser <keir@xen.org>