Pull up following revision(s) (requested by bouyer in ticket #1026):
sys/arch/xen/xen/xbd_xenbus.c: revision 1.43 via patch
sys/arch/xen/xen/xbdback_xenbus.c: revision 1.25
xbdback: implement and publish "feature-flush-cache".
xbd: if feature-flush-cache is present, use it for DIOCCACHESYNC.
If not present, make DIOCCACHESYNC return EOPNOTSUPP and warn on
first call.
Should improve WAPBL reliability of Xen guests on a NetBSD dom0.
Unfortunably not all linux guests seems to support this feature, and using
feature-write-barrier would require a B_BARRIER flag in the buffer.

(snj)