Pull up following revision(s) (requested by jdolecek in ticket #909):

sys/arch/xen/xen/xennetback_xenbus.c: revision 1.64 (patch)

Fix panic of DOM0 in xennetback_xenbus_destroy() on xl destroy of
DOMU with created, but non CONNECTED xennet (such as when DOMU
panics during boot); only try to disestablish the intr if it was
actually setup.

While here protect xnetback_instances with mutex, and switch to use
kmem_zalloc() + KM_SLEEP / kmem_free() like xbdback_xenbus.c; add XXXSMP
to the other global variables, and at least mark them static

(martin)