Simplify how ephemeral disks are created and named.

Resolves bug 1087028.

Change-Id: Ife7ec43cf76cf04a98f1db2135f9149a2eefb55d

nova/tests/test_libvirt.py

@@ -2295,6 +2295,9 @@ class LibvirtConnTestCase(test.TestCase):
         self.stubs.Set(imagebackend.Image,
                        'cache',
                        fake_none)
+        self.stubs.Set(libvirt_driver.LibvirtDriver,
+                       '_create_ephemeral',
+                       fake_none)
 
         conn.spawn(self.context, instance, None, [], 'herp',
                        network_info=network_info)