I'm still totally against this. FD passing is a nice feature for sandboxing,
but the passing should be between closely cooperating programs. We'll
need a tool shipped from the qemu source tree to open and set up the
FDs, and not someone external. With that setup in place we can use
a protocol similar to the various OpenBSD privilegue separated deaemons
to also allow reopening / snapshots / etc.
Opening fds in libvirt and passing them into qemu is exactly the wrong way,
and just cements the current horrors where libvirt duplicates parsing
of image format headers.