If zero clusters are erroneously treated as unallocated, "qemu-img rebase"
will copy the backing file's contents onto the cluster.

The bug existed also in image streaming, but since the root cause was in
qcow2's is_allocated implementation it is enough to test it with qemu-img.
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>