Log DB exception if VIF creation fails

If there is a DBError raised during VIF creation it is nearly impossible
to debug since all exception information is lost. This logs the
exception to aid in debugging. Note that this exception should only be
raised if something is really wrong so this should not spam a normal
setup.

Change-Id: Ifed88c981cc9d564a7443744bf298677a0782cee

nova/db/sqlalchemy/api.py

@@ -1521,6 +1521,7 @@ def virtual_interface_create(context, values):
         vif_ref.update(values)
         vif_ref.save(context.session)
     except db_exc.DBError:
+        LOG.exception(_LE("VIF creation failed with a database error."))
         raise exception.VirtualInterfaceCreateException()
 
     return vif_ref