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 patches 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: Anthony Liguori <aliguori@us.ibm.com>


git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@6910 c046a42c-6fe2-441c-8c8c-71466251a162