qemu: hotplug: Detect disk backing images before setting up security access

qemu: hotplug: Detect disk backing images before setting up security access

The VM will require access also to the detected images. Unfortunately a
recent reordering of the code introduced a bug where the backing chain
was probed after setting up cgroups/selinux/namespaces, which caused
that any detected images were not allowed/added and qemu was then not
able to use them.

Fixes: 9b8bb536ff999fa61e41869bd98a026b8e23378f
Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>