element. Reject 'telnets' and 'tls'. TLS transport for qemu chardevs is
configured via "tls='yes'" attribute added to the "<source>" element
instead, so this prevents potential misconfig as the value would be
silently accepted.
Closes: https://gitlab.com/libvirt/libvirt/-/issues/412 Signed-off-by: Peter Krempa <pkrempa@redhat.com> Reviewed-by: Ján Tomko <jtomko@redhat.com>