Better error message if new VM Name starts with dot

follow-up fix: QubesOS/qubes-issues#7234

qubes/vm/__init__.py

@@ -52,13 +52,13 @@ def validate_name(holder, prop, value):
         raise qubes.exc.QubesValueError(
             'VM name must be shorter than 32 characters')
 
-    if re.match(r"\A[0-9_-].*\Z", value) is not None:
+    if re.match(r"\A[0-9_.-].*\Z", value) is not None:
         if holder is not None and prop is not None:
             raise qubes.exc.QubesPropertyValueError(holder, prop, value,
-                '{} cannot start with hyphen, underscore or numbers'.format(
-                    prop.__name__))
+                '{} cannot start with hyphen, underscore, dot ' \
+                'or numbers'.format(prop.__name__))
         raise qubes.exc.QubesValueError(
-            'VM name cannot start with hyphen, underscore or numbers')
+            'VM name cannot start with hyphen, underscore, dot or numbers')
 
     # this regexp does not contain '+'; if it had it, we should specifically
     # disallow 'lost+found' #1440