ms_driver: Add a virtual bts and make use of it

It's very close to the osmo-bts-trx but without osmo-trx. Modify
the suite to make use of this BTS.

Change-Id: I9f5a2501eb4473ccf2287c902ee207c6a45a1bc5
diff --git a/example/defaults.conf b/example/defaults.conf
index eee17b1..3de5e29 100644
--- a/example/defaults.conf
+++ b/example/defaults.conf
@@ -84,3 +84,6 @@
   type: uhd
   launch_trx: true
   clock_reference: internal
+
+osmo_bts_virtual:
+  max_trx: 1