libvirt: configuration element for a random number generator device

This configuration exposes a paravirtual random number generator device,
with a default backend set to a hosts /dev/random

An optional rate element can be provided in order to limit the consumption
of host's entropy by the guest.

Partially implements blueprint libvirt-rng-device
Change-Id: Id1be32a307473aec2ff0b54b43b2ca3547d74b84

nova/tests/virt/libvirt/test_libvirt_config.py

@@ -1586,3 +1586,27 @@ class LibvirtConfigGuestSeclabel(LibvirtConfigBaseTest):
               <seclabel type='dynamic'>
                 <baselabel>system_u:system_r:my_svirt_t:s0</baselabel>
               </seclabel>""")
+
+
+class LibvirtConfigGuestRngTest(LibvirtConfigBaseTest):
+
+    def test_config_rng_driver(self):
+        obj = config.LibvirtConfigGuestRng()
+
+        xml = obj.to_xml()
+        self.assertXmlEqual(xml, """
+<rng model='virtio'>
+    <backend model='random'>/dev/random</backend>
+</rng>""")
+
+    def test_config_rng_driver_with_rate(self):
+        obj = config.LibvirtConfigGuestRng()
+        obj.rate_period = '12'
+        obj.rate_bytes = '34'
+
+        xml = obj.to_xml()
+        self.assertXmlEqual(xml, """
+<rng model='virtio'>
+    <rate period='12' bytes='34'/>
+    <backend model='random'>/dev/random</backend>
+</rng>""")

nova/virt/libvirt/config.py

@@ -1335,3 +1335,33 @@ class LibvirtConfigNodeDevicePciSubFunctionCap(LibvirtConfigObject):
                                           c.get('bus'),
                                           c.get('slot'),
                                           c.get('function')))
+
+
+class LibvirtConfigGuestRng(LibvirtConfigGuestDevice):
+
+    def __init__(self, **kwargs):
+        super(LibvirtConfigGuestRng, self).__init__(root_name="rng",
+                                                      **kwargs)
+
+        self.model = 'random'
+        self.backend = '/dev/random'
+        self.rate_period = None
+        self.rate_bytes = None
+
+    def format_dom(self):
+        dev = super(LibvirtConfigGuestRng, self).format_dom()
+        dev.set('model', 'virtio')
+
+        backend = etree.Element("backend")
+        backend.set("model", self.model)
+        backend.text = self.backend
+
+        if self.rate_period and self.rate_bytes:
+            rate = etree.Element("rate")
+            rate.set("period", str(self.rate_period))
+            rate.set("bytes", str(self.rate_bytes))
+            dev.append(rate)
+
+        dev.append(backend)
+
+        return dev