core: forbid 'lost+found' as VM name

When /var/lib/qubes/appvms is a mount point of ext4 filesystem, there
will be already 'lost+found' directory. Avoid this conflict.

Fixes QubesOS/qubes-issues#1440

core-modules/000QubesVm.py

@@ -566,6 +566,10 @@ class QubesVm(object):
             return False
         if len(name) > 31:
             return False
+        if name == 'lost+found':
+            # avoid conflict when /var/lib/qubes/appvms is mounted on
+            # separate partition
+            return False
         return re.match(r"^[a-zA-Z][a-zA-Z0-9_.-]*$", name) is not None
 
     def pre_rename(self, new_name):