qemu: hotplug: Reorder setup of disk backend metadata

qemu: hotplug: Reorder setup of disk backend metadata

The regular VM startup code first calls the setup of the disk backing
chain as defined in the XML and then calls the function to load the
rest of the backing chain from the image metadata. The hotplug code
did it the other way around, thus causing a failure when attempting
to attach a QCOW2 image via FD passing.

Reorder the hotplug code to have the same order.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=2193315
Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>