xen: events: add event channel allocation for domain-0

This commit adds interface for evtchn allocation that can be used by
privilaged domain. Domain 0 can specify both dom and remote_dom
parameters for hypercall, where in others domains dom should be always
DOMID_SELF. It is needed for creating pre-defined channels during
domain setup in Zephyr Dom0.

Signed-off-by: Dmytro Firsov <dmytro_firsov@epam.com>