block: failed qemu-img command should return non-zero exit code

block: failed qemu-img command should return non-zero exit code

If the backing file cannot be opened when doing qemu-img rebase, the
variable 'ret' was not assigned a non-zero value, and the qemu-img
process terminated with exit code zero. Fix this.

Signed-off-by: Xu Tian <xutian@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>