qemu-img: add dirty flag status

Some block drivers can verify their image files are clean or not. So we can show
it while using "qemu-img info".

Signed-off-by: Dong Xu Wang <wdongxu@linux.vnet.ibm.com>
Reviewed-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
