qmu-img: fix qemu-img convert to generate a valid image when the source referenced a backing file

Make 'qemu-img convert' copies unallocated parts of the source image
when -B option was not specified.

Signed-off-by: Akkarit Sangpetch <asangpet@andrew.cmu.edu>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
