Add flush support for qcow
Add flush support for qcow

Adds flush support to the qcow handler.
Removes O_SYNC flag and unconditionally sets WCE=1 for qcow.

Signed-off-by: Bryant G. Ly <1024ba5f11bcf6a7ffb2ce4b8b54f508dfeeefbd@linux.vnet.ibm.com>
Signed-off-by: Brad Warrum <0f08a3f14f9f86d2a60b4167f078730120f3a368@us.ibm.com>

open-iscsi/tcmu-runner,open-iscsi/tcmu-runner,agrover/tcmu-runner