Introduce bdrv_check (Kevin Wolf)

From: Kevin Wolf <kwolf@redhat.com>

Introduce a new bdrv_check function pointer for block drivers. Modify qcow2 to
return an error status in check_refcounts(), so it can implement bdrv_check.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>


git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@7214 c046a42c-6fe2-441c-8c8c-71466251a162
