xen/intr: rework xen_intr_alloc_isrc() locking

Locking for allocation was being done in xen_intr_bind_isrc(), but the
unlock was inside xen_intr_alloc_isrc(). While the lock acquisition at
the end of xen_intr_alloc_isrc() was to modify xen_intr_port_to_isrc[],
NOT allocation. Fix this garbled (though working) locking scheme.

Now locking for allocation is strictly in xen_intr_alloc_isrc(), while
locking to modify xen_intr_port_to_isrc[] is in xen_intr_bind_isrc().

Reviewed by: royger
Differential Revision: https://reviews.freebsd.org/D30726