The actual disk space used by an image can vary between filesystems and
depending on other settings like an extent size hint. Replace the one
call of "$QEMU_IMG info" and the associated one-off sed filter with the
more standard "_img_info" and the standard filter from common.filter.

Apart from turning "vmdk" into "IMGFMT" and changing the placeholder for
cid fields, this only removes the "disk size" line.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>