qemu-img: allow qemu-img measure --object without a filename

qemu-img: allow qemu-img measure --object without a filename

In most qemu-img sub-commands the --object option only makes sense when
there is a filename.  qemu-img measure is an exception because objects
may be referenced from the image creation options instead of an existing
image file.  Allow --object without a filename.

Signed-off-by: Stefan Hajnoczi <[email protected]>
Reviewed-by: Max Reitz <[email protected]>
Message-Id: <20200221112522.1497712[email protected]>
Signed-off-by: Max Reitz <[email protected]>