xen/intr: move evtchn_type to intr-internal.h

The evtchn_type enum is only touched by the Xen interrupt code. Other
event channel uses no longer need the value, so that has been moved to
restrict its use.

Copyright note. The current evtchn_type was introduced at 76acc41fb7c7
by Justin T. Gibbs. This in turn appears to have been heavily inspired
by 30d1eefe3937 done by Kip Macy.

Reviewed by: royger