libvirt-sys-vm-importer: Adapt domain's disk config
libvirt-sys-vm-importer: Adapt domain's disk config

When importing domains from system libvirt to session, we convert the
disk contents to our default format (qcow2) so it's wrong to copy the
original disk configuration verbatim. This patch fixes this issue by
removing the copied domain disk config and adding a new one (just like
we do for new VMs we create against installers.

The code also ensures that newly added main disk doesn't conflict with
any existing CD-ROM devices in domain configuration.

vbenes/gnome-boxes,vbenes/gnome-boxes