In some cases backing chain needs to be cleared prior to re-detection.
Move this step out of qemuDomainDetermineDiskChain as only certain
places need it and the function itself is able to skip to the end of the
chain to perform detection.
Signed-off-by: NPeter Krempa <pkrempa@redhat.com>
Reviewed-by: NJán Tomko <jtomko@redhat.com>