Improve developer docs to turn off libvirt

Tell folks to turn off libvirt on their host.

Change-Id: I4ca9a466cd14bd7a88880299f56ea0eba343b6c8

docs/developer-env.md

@@ -54,6 +54,11 @@ Next, download and run the Docker 1.6 LKCD4 (Docker Inc Employee) built binary:
     mv docker-1.6.-rc1 docker
     sudo ./docker -d &
 
+Finally stop libvirt on the host machine.  Only one copy of libvirt may be
+running at a time.
+
+    service libvirt stop
+
 The basic starting environment will be created using `docker-compose`.
 This environment will start up the openstack services listed in the
 compose directory.