vm/qubesvm: improve name property setter

Split it into two functions: validate_name - context-less verification,
and actual _setter_name which perform additional verification in
context of actual VM.
Switch to qubes.exc.* exceptions where appropriate.

qubes/vm/qubesvm.py

@@ -75,20 +75,35 @@ def _setter_qid(self, prop, value):
     return value
 
 
-def _setter_name(self, prop, value):
-    ''' Helper for setting the domain name '''
+def validate_name(holder, prop, value):
+    ''' Check if value is syntactically correct VM name '''
     if not isinstance(value, str):
-        raise TypeError('{} value must be string, {!r} found'.format(
-            prop.__name__, type(value).__name__))
+        raise TypeError('VM name must be string, {!r} found'.format(
+            type(value).__name__))
     if len(value) > 31:
-        raise ValueError('{} value must be shorter than 32 characters'.format(
-            prop.__name__))
+        if holder is not None and prop is not None:
+            raise qubes.exc.QubesPropertyValueError(holder, prop, value,
+                '{} value must be shorter than 32 characters'.format(
+                    prop.__name__))
+        else:
+            raise qubes.exc.QubesValueError(
+                'VM name must be shorter than 32 characters')
 
     # this regexp does not contain '+'; if it had it, we should specifically
     # disallow 'lost+found' #1440
     if re.match(r"^[a-zA-Z][a-zA-Z0-9_-]*$", value) is None:
-        raise ValueError('{} value contains illegal characters'.format(
-            prop.__name__))
+        if holder is not None and prop is not None:
+            raise qubes.exc.QubesPropertyValueError(holder, prop, value,
+                '{} value contains illegal characters'.format(prop.__name__))
+        else:
+            raise qubes.exc.QubesValueError(
+                'VM name contains illegal characters')
+
+
+def _setter_name(self, prop, value):
+    ''' Helper for setting the domain name '''
+    validate_name(self, prop, value)
+
     if self.is_running():
         raise qubes.exc.QubesVMNotHaltedError(
             self, 'Cannot change name of running VM')
@@ -101,7 +116,7 @@ def _setter_name(self, prop, value):
         pass
 
     if value in self.app.domains:
-        raise qubes.exc.QubesValueError(
+        raise qubes.exc.QubesPropertyValueError(self, prop, value,
             'VM named {} alread exists'.format(value))
 
     return value