Trivial: use string for exception message

Exception messages are expected to be strings. Some Nova exception
handlers will fail when encountering wrapped exceptions.

This change fixes the only occurrence of this issue.

Change-Id: If35f7b99f9c0b9f0edd614dc96842549e6ac19c1
Closes-Bug: #1801713

compute_hyperv/nova/vmops.py

@@ -1096,7 +1096,8 @@ class VMOps(object):
                 except Exception as exc:
                     LOG.exception("Failed to plug vif: '%s'.",
                                   vif, instance=instance)
-                    raise exception.VirtualInterfacePlugException(exc)
+                    raise exception.VirtualInterfacePlugException(
+                        six.text_type(exc))
 
     def unplug_vifs(self, instance, network_info):
         if network_info: