core: handle errors with xenstore access during VM shutdown

When netvm and firewallvm is shut down, netvm handling code will
try to revoke firewallvm access to external IP. But if netvm shutdown
happens in the meantime, xenstore will throw ENOENT error.

core-modules/005QubesNetVm.py

@@ -22,6 +22,7 @@
 #
 import sys
 import os.path
+import xen.lowlevel.xs
 
 from qubes.qubes import QubesVm,register_qubes_vm_class,xs,dry_run
 from qubes.qubes import defaults,system_path,vm_files
@@ -120,8 +121,12 @@ class QubesNetVm(QubesVm):
         for xid in self.__external_ip_allowed_xids:
             perms.append({ 'dom': xid, 'read': True })
 
-        xs.set_permissions('', '/local/domain/{0}/qubes-netvm-external-ip'.format(xid),
-                perms)
+        try:
+            xs.set_permissions('', '/local/domain/{0}/qubes-netvm-external-ip'.format(xid),
+                    perms)
+        except xen.lowlevel.xs.Error as e:
+            print >>sys.stderr, "WARNING: failed to update external IP " \
+                                "permissions: %s" % (str(e))
 
     def start(self, **kwargs):
         if dry_run: