tests: Add iotest mirror-sparse for recent patches

tests: Add iotest mirror-sparse for recent patches

Prove that blockdev-mirror can now result in sparse raw destination
files, regardless of whether the source is raw or qcow2.  By making
this a separate test, it was possible to test effects of individual
patches for the various pieces that all have to work together for a
sparse mirror to be successful.

Note that ./check -file produces different job lengths than ./check
-qcow2 (the test uses a filter to normalize); that's because when
deciding how much of the image to be mirrored, the code looks at how
much of the source image was allocated (for qcow2, this is only the
written clusters; for raw, it is the entire file).  But the important
part is that the destination file ends up smaller than 3M, rather than
the 20M it used to be before this patch series.

Signed-off-by: Eric Blake <eblake@redhat.com>
Message-ID: <20250509204341.3553601-28-eblake@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>