Fixes qemu-img create by adding backing file format

Not providing backing_fmt to the qemu-create when creating qcow
images results in failure. Fixed by finding the file format and
adding the backing_fmt argument.
Merge pull request #13 from sthaha/b30dea5a718eda7026ea9c0fb5474e66522d0e9d

Fixes broken creation of cow images based on other qcow image