Add a test for the new blkdebug/blkverify interface.

This test is not written in Python, although it uses QMP. This is
because it invokes the qemu-io HMP command, which outputs errors to
stderr instead of returning them through QMP. Filtering and testing that
output is easier in a shell script than with the Python infrastructure.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>