Properly mark auto-added 'terminator' virStorageSource

Properly mark auto-added 'terminator' virStorageSource

All backing chain members which were auto-added by image detection,
including the terminating element, should have the 'detected' property
set to true. This is needed to properly strip the detected elements in
some cases, e.g. for the status XML where we could treat some images as
manually terminated even when it was auto-detected.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>