Allow relative path for qemu backing file

Allow relative path for qemu backing file

This patch enables the relative backing file path support provided by
qemu-img create.

If a relative path is specified for the backing file, it is converted
to an absolute path using the storage pool path. The absolute path is
used to verify that the backing file exists. If the backing file exists,
the relative path is allowed and will be provided to qemu-img create.