domain: Allow overriding RNG device through env

On some systems (e.g Ubuntu-using travis runner), `/dev/urandom` isn't
available it seems to let's allow overriding the RNG device through an
environment variable, `TF_LIBVIRT_RNG_DEV`.

Author: zeenix

CONTRIBUTING.md

@@ -74,6 +74,13 @@ variable:
 LIBVIRT_DEFAULT_URI='qemu+unix:///session' TF_LIBVIRT_DISABLE_PRIVILEGED_TESTS=1 make testacc
 ```
 
+If '/dev/random' is not available on the platform you run the acceptance tests on, you can override the device used
+through an environment variable as well:
+
+```bash
+TF_LIBVIRT_RNG_DEV='/dev/random' make testacc
+```
+
 ### Code coverage:
 
 Run first the testacc suite.

libvirt/domain_def.go

@@ -69,14 +69,6 @@ func newDomainDef() libvirtxml.Domain {
 					},
 				},
 			},
-			RNGs: []libvirtxml.DomainRNG{
-				{
-					Model: "virtio",
-					Backend: &libvirtxml.DomainRNGBackend{
-						Random: &libvirtxml.DomainRNGBackendRandom{Device: "/dev/urandom"},
-					},
-				},
-			},
 		},
 		Features: &libvirtxml.DomainFeatureList{
 			PAE:  &libvirtxml.DomainFeature{},
@@ -91,6 +83,21 @@ func newDomainDef() libvirtxml.Domain {
 		domainDef.Type = "kvm"
 	}
 
+	// FIXME: We should allow setting this from configuration as well.
+	rngDev := os.Getenv("TF_LIBVIRT_RNG_DEV")
+	if rngDev == "" {
+		rngDev = "/dev/urandom"
+	}
+
+	domainDef.Devices.RNGs = []libvirtxml.DomainRNG{
+		{
+			Model: "virtio",
+			Backend: &libvirtxml.DomainRNGBackend{
+				Random: &libvirtxml.DomainRNGBackendRandom{Device: rngDev},
+			},
+		},
+	}
+
 	return domainDef
 }