Force the first character of VM name to be a letter

Especially don't allow numeric-only name (our deserializer of qubes.xml
will convert it to int instead of str...).

core-modules/000QubesVm.py

@@ -399,7 +399,7 @@ class QubesVm(object):
             return False
 
     def verify_name(self, name):
-        return re.match(r"^[a-zA-Z0-9_-]*$", name) is not None
+        return re.match(r"^[a-zA-Z][a-zA-Z0-9_-]*$", name) is not None
 
     def pre_rename(self, new_name):
         # fire hooks