block/iscsi: optimize WRITE10/16 if cache.writeback is not set

block/iscsi: optimize WRITE10/16 if cache.writeback is not set

SCSI allowes to tell the target to not return from a write command
if the date is not written to the disk. Use this so called FUA
bit if it is supported to optimize WRITE commands if writeback is
not allowed.

In this case qemu always issues a WRITE followed by a FLUSH. This
is 2 round trip times. If we set the FUA bit we can ignore the
following FLUSH.

Signed-off-by: Peter Lieven <pl@kamp.de>
Message-id: 1429193313-4263-6-git-send-email-pl@kamp.de
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>