Draw no buoy for a virtual aid to navigation

A virtual aid (AIS message 21's VirtualAtoN flag) is a mark broadcast
where nothing physical stands, so drawing it as a buoy showed a mark
that is not there. The vessel source record now carries `virtual`
(false unless a source says true; any source may set it), AISStream
sets it from VirtualAtoN, and the vessel layer keeps such an aid's
disc: vesselModelSpec returns null for it and the layer gathers no
model item for it. Its card, selected card and readout add VIRTUAL
after its kind.

src/data/vesselTypes.test.mjs

@@ -36,6 +36,11 @@ test('aids to navigation are known by their type, else their MMSI', () => {
     'Emergency wreck marking buoy',
   ])
     assert.equal(isAidToNavigation({ type, mmsi: '' }), true, type);
+  // A virtual aid is an aid whatever its type or MMSI.
+  assert.equal(
+    isAidToNavigation({ type: '', mmsi: '', virtual: true }),
+    true,
+  );
   // AIS aid-to-navigation stations are numbered 99MIDxxxx.
   assert.equal(isAidToNavigation({ type: '', mmsi: '993672001' }), true);
   assert.equal(isAidToNavigation({ type: '0', mmsi: ' 992351234 ' }), true);