Pull up following revision(s) (requested by jdolecek in ticket #984):

sys/arch/xen/xen/xbd_xenbus.c: revision 1.83

avoid race condition between I/O submission in xbd_diskstart() and
interrupt handling in xbd_handler() - need to protect it with splbio()

fixes PR port-xen/53506 by Emmanuel Dreyfus, and likely also port-xen/53074
by Brad Spencer

(martin)