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/suites/nitb_netreg_mass/suite.conf b/suites/nitb_netreg_mass/suite.conf
index bfb23cd..94ec603 100644
--- a/suites/nitb_netreg_mass/suite.conf
+++ b/suites/nitb_netreg_mass/suite.conf
@@ -2,7 +2,7 @@
   ip_address:
   - times: 1
   bts:
-  - times: 1
+  - type: osmo-bts-virtual
 
 defaults:
   timeout: 40s