iotests: Improve and rename test 291 to qemu-img-bitmap

iotests: Improve and rename test 291 to qemu-img-bitmap

Enhance the test to demonstrate existing less-than-stellar behavior of
qemu-img with a qcow2 image containing an inconsistent bitmap: we
don't diagnose the problem until after copying the entire image (a
potentially long time), and when we do diagnose the failure, we still
end up leaving an empty bitmap in the destination.  This mess will be
cleaned up in the next patch.

While at it, rename the test now that we support useful iotest names,
and fix a missing newline in the error message thus exposed.

Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <20210709153951.2801666-2-eblake@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Nir Soffer <nsoffer@redhat.com>