blkverify: Add block driver for verifying I/O

The blkverify block driver makes investigating image format data
corruption much easier.  A raw image initialized with the same contents
as the test image (e.g. qcow2 file) must be provided.  The raw image
mirrors read/write operations and is used to verify that data read from
the test image is correct.

See docs/blkverify.txt for more information.

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