iotests: Use convert -n in some cases

iotests: Use convert -n in some cases

qemu-img convert (without -n) can often be replaced by a combination of
_make_test_img + qemu-img convert -n.  Doing so allows converting to
protocols that do not allow direct file creation, such as FUSE exports.
The only problem is that for formats other than qcow2 and qed (qcow1 at
least), this may lead to high disk usage for some reason, so we cannot
do it everywhere.

But we can do it in 028 and 089, so let us do that so they can run on
FUSE exports.  Also, in 028 this allows us to remove a 9-line comment
that used to explain why we cannot safely filter drive-backup's image
creation output.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Message-Id: <20201027190600.192171-10-mreitz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>