[Qemu-block] RFC iscsi: set FUA and DPO if !bs->enable_write_cache

[Peter Lieven]

Hi,

Ronnie came up with an idea to reduce latency if !bs->enable_write_cache for an 
iSCSI device.

If !bs->enable_write_cache Qemu sends a flush after every single write. What 
could be done is
the following:

if (!bs->enable_write_cache)
 set FUA (force unit access) and DPO (disable page out) bits in every write cmd
 make iscsi_co_flush a NOOP in this case.

Your thoughts?

Thanks,
Peter