virsh: Remove <backingStore> sub-element in virshFindDisk

virsh: Remove <backingStore> sub-element in virshFindDisk

Previously we've removed the data only in virshUpdateDiskXML when
changing the disk source for the CDROM since the backing store would be
invalid. Move the code into a separate function and callit from
virshFindDisk which is also used when detaching disk.

The detaching code does not necessarily need to get the full backing
chain since it will need to act on the one managed by libvirt anyways
and this also takes care of problems when parts of the backing store
were invalid due to buggy RBD detection code.

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