libvirt: cleanup() serial_consoles after instance failure

When nova cleanup() is called to release all resources related to an instance
it can thow InstanceNotFound.  Which is not caught and therefore returned to
the user.

This is because the generator function (_get_serial_ports_from_instance)
is not *executed* under try/except.  While that is clearly the intent of the
code.

Change-Id: I9e9410ab7eec5fa667c0c3de4548c49df050c167
Closes-Bug: 1480514

nova/tests/unit/virt/libvirt/test_driver.py

@@ -10946,7 +10946,10 @@ class LibvirtConnTestCase(test.NoDBTestCase):
 
         # Ensure _get_serial_ports_from_instance raises same exception
         # that would have occurred if domain was gone.
-        get_ports.side_effect = exception.InstanceNotFound("domain undefined")
+        def exception_with_yield(instance):
+            raise exception.InstanceNotFound("domain undefined")
+            yield
+        get_ports.side_effect = exception_with_yield
 
         drvr = libvirt_driver.LibvirtDriver(fake.FakeVirtAPI())
         drvr.firewall_driver = firewall_driver

nova/virt/libvirt/driver.py

@@ -884,11 +884,10 @@ class LibvirtDriver(driver.ComputeDriver):
         if CONF.serial_console.enabled:
             try:
                 serials = self._get_serial_ports_from_instance(instance)
+                for hostname, port in serials:
+                    serial_console.release_port(host=hostname, port=port)
             except exception.InstanceNotFound:
-                # Serial ports already gone. Nothing to release.
-                serials = ()
-            for hostname, port in serials:
-                serial_console.release_port(host=hostname, port=port)
+                pass
 
         self._undefine_domain(instance)