qemu: block: Unify conditions to format backing store of format node definition
qemu: block: Unify conditions to format backing store of format node definition

Move all bits of the formatting of the 'backing' attribute to a single
condition and make it use a single extracted copy of the backing store.

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

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