qfile-daemon-dvm: Implement LAUNCH and FINISH actions

If the action is LAUNCH instead of qubes.SomeService, then just start
the dispVM, write (only) its name to stdout, and quit.

If the action is FINISH, then kill and remove the named dispVM.

dispvm/qfile-daemon-dvm

@@ -171,6 +171,10 @@ class QfileDaemonDvm:
 
 def main():
     exec_index = sys.argv[1]
+    if exec_index == "FINISH":
+        QfileDaemonDvm.finish_disposable(sys.argv[2])
+        return
+
     src_vmname = sys.argv[2]
     user = sys.argv[3]
     # accessed directly by get_dvm()
@@ -183,6 +187,10 @@ def main():
     qfile = QfileDaemonDvm(src_vmname)
     dispvm = qfile.get_dvm()
     if dispvm is not None:
+        if exec_index == "LAUNCH":
+            print dispvm.name
+            return
+
         print >>sys.stderr, "time=%s, starting VM process" % (str(time.time()))
         subprocess.call(['/usr/lib/qubes/qrexec-client', '-d', dispvm.name,
                          user+':exec /usr/lib/qubes/qubes-rpc-multiplexer ' +