libxl,xl: add max_event_channels option to xl configuration file

libxl,xl: add max_event_channels option to xl configuration file

Add the 'max_event_channels' option to the xl configuration file to
limit the number of event channels that domain may use.

Plumb this option through to libxl via a new libxl_build_info field
and call xc_domain_set_max_evtchn() in the post build stage of domain
creation.

A new LIBXL_HAVE_BUILDINFO_EVENT_CHANNELS #define indicates that this
new field is available.

The default value of 1023 limits the domain to using the minimum
amount of global mapping pages and at most 5 xenheap pages.

Signed-off-by: David Vrabel <david.vrabel@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Keir Fraser <keir@xen.org>