iotests: Check dirty bitmap statistics in 124

We had a bug for multiple releases where dirty-bitmap count was
documented in bytes but reported in sectors; enhance the testsuite
to add coverage of DirtyBitmapInfo to ensure we do not regress again.

Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: John Snow <jsnow@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
