qemu-iotests: Test query-blockstats with -drive and -blockdev

Make sure that query-blockstats returns information for every
BlockBackend that is named or attached to a device model (or both).

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>