Avoid TypeError when loading libvirt.LibvirtDriver

Fixes bug 1078304

The libvirt.LibvirtDriver.__init__() signature has changed since:

  openstack/nova@081b652c

to take a new virtapi parameter. This is not required by any
ceilometer usage of the driver, however the absence of a
corresponding argument causes importutils.import_object_ns()
to fail with TypeError.

For testing the nova fake.FakeVirtAPI still uses the old
signature, hence we try both.

Change-Id: I05a9728ceb435a72f5fa8a648cd70b079126396e

ceilometer/compute/libvirt.py

@@ -44,8 +44,13 @@ def get_libvirt_connection():
     # The direct-import implementation only works with Folsom because
     # the configuration setting changed.
     try:
-        return importutils.import_object_ns('nova.virt',
-                                            FLAGS.compute_driver)
+        try:
+            return importutils.import_object_ns('nova.virt',
+                                                FLAGS.compute_driver,
+                                                None)
+        except TypeError:
+            return importutils.import_object_ns('nova.virt',
+                                                FLAGS.compute_driver)
     except ImportError:
         # Fall back to the way it was done in Essex.
         import nova.virt.connection