qemu-iotests: Add test case for mirror with unmap

This checks that the discard on mirror source that effectively zeroes
data is also reflected by the data of target.

Signed-off-by: Fam Zheng <famz@redhat.com>
Reviewed-by: John Snow <jsnow@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>