qemu-img: adding a "-F base_fmt" option to "qemu-img create -b"

qemu-img: adding a "-F base_fmt" option to "qemu-img create -b"

If the user specifies the backing file format,
then when opening the backing file, there is no need
to probe the (backing file) image to figure out its format.

This follows my previous patch implementing bdrv_create2
which keeps (for qcow2 only) the backing file format
as a qcow2-extension

Suggested by Daniel P. Berrange.

Signed-off-by: Uri Lublin <uril@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
RH-Upstream-status: pending
Bugzilla: 484080
Acked-by: Eduardo Habkost <ehabkost@redhat.com>
Acked-by: Dor Laor <dlaor@redhat.com>