xen/intr: remove type argument from xen_intr_alloc_isrc()

This value doesn't need to be set in xen_intr_alloc_isrc(). What is
needed is simply to ensure the allocated xenisrc won't appear as free,
even if xi_type is written non-atomically. Since the type is no longer
used to indicate free or not, the calling function should take care of
all non-architecture initialization.

Reviewed by: royger
Differential Revision: https://reviews.freebsd.org/D31188