libvhost-user-glib: use g_main_context_get_thread_default()

If we use NULL, we just get the main program default mainloop
here. Using g_main_context_get_thread_default() has basically
the same effect, but it lets us start different devices in
different threads with different mainloops, which can be useful.

Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Message-Id: <20200123081708.7817-4-johannes@sipsolutions.net>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

contrib/libvhost-user/libvhost-user-glib.c

@@ -89,7 +89,7 @@ vug_source_new(VugDev *gdev, int fd, GIOCondition cond,
     src->gfd.events = cond;
 
     g_source_add_poll(gsrc, &src->gfd);
-    id = g_source_attach(gsrc, NULL);
+    id = g_source_attach(gsrc, g_main_context_get_thread_default());
     g_assert(id);
 
     return gsrc;