If the user doesn't specify a format (ie. they want autodetection),
and the drive is set to read-only, previously we created a qcow2
overlay to protect the drive but didn't set the backing_fmt field.
However libvirt disabled this so now libvirt doesn't work at all on
qcow2 files that have no backing_fmt field set.
We must therefore do autodetection in libguestfs and use that to set
the backing_fmt field.
We were already doing autodetection in the non-readonly case, so this
commit refactors that code into a new function
("get_source_format_or_autodetect") and uses it on the readonly path
too.
c8c181e8d9