While the VM is running the persistent source of a disk might differ
e.g. as the 'newDef' was redefined. Our snapshot code would blindly
rewrite the source of such disk if it shared the 'target'. Fix this by
checking whether the source is the same in the first place.
Signed-off-by: NPeter Krempa <pkrempa@redhat.com>
Reviewed-by: NJán Tomko <jtomko@redhat.com>