Added a backing_format field to BlockDriverState.
Added bdrv_create2 and drv->bdrv_create2 to create an image with
a known backing file format.
Upon bdrv_open2 if backing format is known use it, instead of
probing the (backing) image.
Signed-off-by: NUri Lublin <uril@redhat.com>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>


git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@6908 c046a42c-6fe2-441c-8c8c-71466251a162