qmp: Add optional bool "unmap" to drive-mirror

If specified as "true", it allows discarding on target sectors where source is
not allocated.

Signed-off-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
(cherry picked from commit 0fc9f8ea)

* added to maintain any interdependencies between patches in the
  set. not intended as a new feature for 2.3.1, though it's there
  for anyone interested

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