qemu-iotests: add tests for rebasing zero clusters

qemu-iotests: add tests for rebasing zero clusters

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: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
(cherry picked from commit acbf30ec601b1f817febc4500025b7c4181312c4)

Conflicts:

	tests/qemu-iotests/group

* fixed up to account for tests 48/49 being missing from 1.4

Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>