Need to reset device model pid on reboot

Without this, xend will not spawn a new device model process after the reboot.

Signed-off-by: Ke Yu <ke.yu@intel.com>
Signed-off-by: Arun Sharma <arun.sharma@intel.com>

tools/python/xen/xend/XendDomainInfo.py

@@ -642,6 +642,7 @@ class XendDomainInfo:
                 pass
         if self.image:
             try:
+                self.device_model_pid = 0
                 self.image.destroy()
                 self.image = None
             except: