hvm: update appmenus only on the first qubes.NotifyTools call

Or more precisely - only when qrexec_installed value is changed from
False to True.
Previously it was called at each VM startup.

qubes-rpc/qubes-notify-tools

@@ -14,6 +14,7 @@ def main():
         print >> sys.stderr, 'This script must be called as qrexec service!'
         exit(1)
 
+    prev_qrexec_installed = False
     source_vm = None
     qvm_collection = QubesVmCollection()
     qvm_collection.lock_db_for_writing()
@@ -66,6 +67,7 @@ def main():
         else:
             user = None
 
+        prev_qrexec_installed = source_vm.qrexec_installed
         # Let the tools to be able to enable *or disable* each particular component
         source_vm.qrexec_installed = qrexec > 0
         source_vm.guiagent_installed = gui > 0
@@ -81,11 +83,12 @@ def main():
     finally:
         qvm_collection.unlock_db()
 
-    retcode = subprocess.call(['qvm-sync-appmenus', '--force-rpc'])
-    if retcode == 0 and hasattr(source_vm, 'appmenus_recreate'):
-        # TODO: call the same for child VMs? This isn't done for Linux VMs,
-        # so probably should be ignored for Windows also
-        source_vm.appmenus_recreate()
+    if not prev_qrexec_installed and source_vm.qrexec_installed:
+        retcode = subprocess.call(['qvm-sync-appmenus', '--force-rpc'])
+        if retcode == 0 and hasattr(source_vm, 'appmenus_recreate'):
+            # TODO: call the same for child VMs? This isn't done for Linux VMs,
+            # so probably should be ignored for Windows also
+            source_vm.appmenus_recreate()
 
 
 main()