Libvirt doesn't reliably know the location of the backing chain when
pre-creating images for non-shared migration. This isn't a problem for
full copy, but incremental copy requires the information.

Forbid pre-creating the image in cases where incremental migration is
required. This limitation can perhaps be lifted once libvirt will fully
support loading of backing chain information from the XML.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1249587