xen/blkfront: cope with backend that fail empty BLKIF_OP_WRITE_BARRIER requests

Some(?) Xen block backends fail BLKIF_OP_WRITE_BARRIER requests, which
Linux uses as a cache flush operation.  In that case, disable use
of FLUSH.

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Cc: Daniel Stodden <daniel.stodden@citrix.com>