qemu-img: Fail creation if backing format is invalid

The qemu-img create command should check the backing format to ensure
only image files with valid backing formats are created.  By checking in
qemu-img.c we can print a useful error message.

Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
