qemu: block: Properly format storage slice into backing store strings
qemu: block: Properly format storage slice into backing store strings

When creating overlay images e.g. for snapshots or when merging
snapshots we often specify the backing store string to use. Make the
formatter aware of backing chain entries which have a <slice>
configured so that we record it properly. Otherwise such images
would not work without the XML (when detecting the backing chain).

Signed-off-by: Peter Krempa <2cf5c04c61aa466e4a47bfedc747d17279c72ffc@redhat.com>
Reviewed-by: Ján Tomko <4cab11cfb98d3c937327354a78eb07dbb6ee2bc6@redhat.com>

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