xen/intr: use struct xenisrc * as xen_intr_handle_t

Since xen_intr_handle_t is meant to be an opaque handle and the only
use is retrieving the associated struct xenisrc *, directly use it as
the opaque handler.

Also add a wrapper function for converting the other direction. If some
other value becomes appropriate in the future, these two functions will
be the only spots needing modification.

Reviewed by: mhorne, royger
Differential Revision: https://reviews.freebsd.org/D29500