libvirt: remove explicit /dev/random rng default

libvirt (or rather qemu) will default to /dev/random if no rng device
path is specified i.e.

  <rng model='virtio'>
    <backend model='random'/>
  </rng>

It's preferable for us to not duplicate this default to allow for a
future where libvirt or the hypervisor needs to make more intelligent
decisions about the default device to use.

Also, improve the unit tests to catch this issue.

Change-Id: Ia0f922f7de575496972b4dfa9d56aa22205f1758

nova/tests/virt/libvirt/test_libvirt.py

@@ -1387,6 +1387,10 @@ class LibvirtConnTestCase(test.TestCase):
                                   vconfig.LibvirtConfigGuestVideo)
             self.assertIsInstance(cfg.devices[6],
                                   vconfig.LibvirtConfigGuestRng)
+            self.assertEqual(cfg.devices[6].model, 'random')
+            self.assertIsNone(cfg.devices[6].backend)
+            self.assertIsNone(cfg.devices[6].rate_bytes)
+            self.assertIsNone(cfg.devices[6].rate_period)
 
     def test_get_guest_config_with_rng_not_allowed(self):
         self.flags(virt_type='kvm',
@@ -1453,6 +1457,8 @@ class LibvirtConnTestCase(test.TestCase):
                                   vconfig.LibvirtConfigGuestVideo)
             self.assertIsInstance(cfg.devices[6],
                                   vconfig.LibvirtConfigGuestRng)
+            self.assertEqual(cfg.devices[6].model, 'random')
+            self.assertIsNone(cfg.devices[6].backend)
             self.assertEqual(cfg.devices[6].rate_bytes, 1024)
             self.assertEqual(cfg.devices[6].rate_period, 2)
 
@@ -1495,7 +1501,10 @@ class LibvirtConnTestCase(test.TestCase):
                              vconfig.LibvirtConfigGuestVideo)
             self.assertIsInstance(cfg.devices[6],
                              vconfig.LibvirtConfigGuestRng)
+            self.assertEqual(cfg.devices[6].model, 'random')
             self.assertEqual(cfg.devices[6].backend, '/dev/hw_rng')
+            self.assertIsNone(cfg.devices[6].rate_bytes)
+            self.assertIsNone(cfg.devices[6].rate_period)
 
     def test_get_guest_config_with_rng_dev_not_present(self):
         self.flags(virt_type='kvm',

nova/tests/virt/libvirt/test_libvirt_config.py

@@ -1616,11 +1616,12 @@ class LibvirtConfigGuestRngTest(LibvirtConfigBaseTest):
         xml = obj.to_xml()
         self.assertXmlEqual(xml, """
 <rng model='virtio'>
-    <backend model='random'>/dev/random</backend>
+    <backend model='random'/>
 </rng>""")
 
     def test_config_rng_driver_with_rate(self):
         obj = config.LibvirtConfigGuestRng()
+        obj.backend = '/dev/random'
         obj.rate_period = '12'
         obj.rate_bytes = '34'
 

nova/virt/libvirt/config.py

@@ -1371,7 +1371,7 @@ class LibvirtConfigGuestRng(LibvirtConfigGuestDevice):
                                                       **kwargs)
 
         self.model = 'random'
-        self.backend = '/dev/random'
+        self.backend = None
         self.rate_period = None
         self.rate_bytes = None