[HVM][LINUX][TOOLS] Make xm {shutdown|reboot} do something sensible for HVM

domains with PV drivers loaded.  This patch creates a new PV-on-HVM
module, reboot.ko, which, when loaded, creates control/reboot_module
in the store.  The tools notice this, and disable the watch which would
normally destroy HVM domains which are the target of an xm shutdown
command, allowing the reboot module to shut the domain down cleanly.

Signed-off-by: Tetsu Yamamoto <yamamoto.tetsu@jp.fujitsu.com>

(Checkin comments by Steven Smith <sos22@cam.ac.uk>)

tools/python/xen/xend/image.py

@@ -282,6 +282,7 @@ class HVMImageHandler(ImageHandler):
         log.debug("apic           = %d", self.apic)
 
         self.register_shutdown_watch()
+        self.register_reboot_module_watch()
 
         return xc.hvm_build(dom            = self.vm.getDomid(),
                             image          = self.kernel,
@@ -416,6 +417,7 @@ class HVMImageHandler(ImageHandler):
 
     def destroy(self):
         self.unregister_shutdown_watch();
+        self.unregister_reboot_module_watch();
         if not self.pid:
             return
         os.kill(self.pid, signal.SIGKILL)
@@ -458,6 +460,39 @@ class HVMImageHandler(ImageHandler):
 
         return 1 # Keep watching
 
+    def register_reboot_module_watch(self):
+        """ add xen store watch on control/reboot_module """
+        self.rebootModuleWatch = xswatch(self.vm.dompath + "/control/reboot_module", \
+                                    self.hvm_reboot_module)
+        log.debug("hvm reboot module watch registered")
+
+    def unregister_reboot_module_watch(self):
+        """Remove the watch on the control/reboot_module, if any. Nothrow
+        guarantee."""
+
+        try:
+            if self.rebootModuleWatch:
+                self.rebootModuleWatch.unwatch()
+        except:
+            log.exception("Unwatching hvm reboot module watch failed.")
+        self.rebootModuleWatch = None
+        log.debug("hvm reboot module watch unregistered")
+
+    def hvm_reboot_module(self, _):
+        """ watch call back on node control/reboot_module,
+            if node changed, this function will be called
+        """
+        xd = xen.xend.XendDomain.instance()
+        vm = xd.domain_lookup( self.vm.getDomid() )
+
+        reboot_module_status = vm.readDom('control/reboot_module')
+        log.debug("hvm_reboot_module fired, module status=%s", reboot_module_status)
+        if reboot_module_status == 'installed':
+            self.unregister_shutdown_watch()
+
+        return 1 # Keep watching
+
+
 class IA64_HVM_ImageHandler(HVMImageHandler):
 
     ostype = "hvm"