Use creation content to signal virtual-ness

This makes things a lot simpler.

src/CallHandler.tsx

@@ -93,7 +93,8 @@ export const PROTOCOL_SIP_NATIVE = 'im.vector.protocol.sip_native';
 export const PROTOCOL_SIP_VIRTUAL = 'im.vector.protocol.sip_virtual';
 
 const CHECK_PROTOCOLS_ATTEMPTS = 3;
-// Event type for room account data used to mark rooms as virtual rooms (and store the ID of their native room)
+// Event type for room account data and room creation content used to mark rooms as virtual rooms
+// (and store the ID of their native room)
 export const VIRTUAL_ROOM_EVENT_TYPE = 'im.vector.is_virtual_room';
 
 enum AudioID {