dlna: fix root XML service descriptor

The SCPD URL was being set after marshalling the XML, and thus coming
out blank.  Now works on my Samsung TV, and likely fixes some issues
reported by others in #2648.

cmd/serve/dlna/dlna_test.go

@@ -59,6 +59,8 @@ func TestRootSCPD(t *testing.T) {
 	// Make sure that the SCPD contains a CDS service.
 	require.Contains(t, string(body),
 		"<serviceType>urn:schemas-upnp-org:service:ContentDirectory:1</serviceType>")
+	// Ensure that the SCPD url is configured.
+	require.Regexp(t, "<SCPDURL>/.*</SCPDURL>", string(body))
 }
 
 // Make sure that it serves content from the remote.