conf: Validate disk source configuration also for the backing store
conf: Validate disk source configuration also for the backing store

Since we already parse the <backingStore> of a disk source, we should
also validate the configuration for the whole backing chain and not only
for the top level image.

Signed-off-by: Peter Krempa <2cf5c04c61aa466e4a47bfedc747d17279c72ffc@redhat.com>

zippy2/libvirt,olafhering/libvirt,jardasgit/libvirt,fabianfreyer/libvirt,zippy2/libvirt,andreabolognani/libvirt,crobinso/libvirt,jardasgit/libvirt,olafhering/libvirt,andreabolognani/libvirt,jardasgit/libvirt,eskultety/libvirt,eskultety/libvirt,zippy2/libvirt,jardasgit/libvirt,jfehlig/libvirt,olafhering/libvirt,jfehlig/libvirt,libvirt/libvirt,zippy2/libvirt,andreabolognani/libvirt,crobinso/libvirt,libvirt/libvirt,fabianfreyer/libvirt,eskultety/libvirt,nertpinx/libvirt,eskultety/libvirt,andreabolognani/libvirt,andreabolognani/libvirt,fabianfreyer/libvirt,crobinso/libvirt,libvirt/libvirt,nertpinx/libvirt,nertpinx/libvirt,jfehlig/libvirt,libvirt/libvirt,crobinso/libvirt,olafhering/libvirt,fabianfreyer/libvirt,jardasgit/libvirt,nertpinx/libvirt,fabianfreyer/libvirt,nertpinx/libvirt,jfehlig/libvirt,eskultety/libvirt