qemu-img: add image fragmentation statistics

Discussion can be found at:
http://patchwork.ozlabs.org/patch/128730/

This patch add image fragmentation statistics while using qemu-img check.

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>
