iothread: create main loop unconditionally

Since we've have the gcontext always there, create the main loop
altogether.  The iothread_run() is even cleaner.

Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Peter Xu <peterx@redhat.com>
Message-id: 20190306115532.23025-4-peterx@redhat.com
Message-Id: <20190306115532.23025-4-peterx@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>