xen/domctl: Merge set_max_evtchn into createdomain

xen/domctl: Merge set_max_evtchn into createdomain

set_max_evtchn is somewhat weird.  It was introduced with the event_fifo work,
but has never been used.  Still, it is a bounding on resources consumed by the
event channel infrastructure, and should be part of createdomain, rather than
editable after the fact.

Drop XEN_DOMCTL_set_max_evtchn completely (including XSM hooks and libxc
wrappers), and retain the functionality in XEN_DOMCTL_createdomain.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Daniel De Graaf <dgdegra@tycho.nsa.gov>
Acked-by: Christian Lindig <christian.lindig@citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>