conf: Drop restrictions on rng backend path

Currently we only allow /dev/random and /dev/hwrng as host input
for <rng><backend model='random'/> device. This was added after
various upstream discussions in commit 4932ef45

However this restriction has generated quite a few complaints over
the years, so a new discussion was initiated:

http://www.redhat.com/archives/libvir-list/2016-April/msg00987.html

Several people suggested removing the restriction, and nobody really
spoke up to defend it. So this patch drops the path restriction
entirely

https://bugzilla.redhat.com/show_bug.cgi?id=1074464

docs/formatdomain.html.in

@@ -6244,10 +6244,10 @@ qemu-kvm -net nic,model=? /dev/null
           <dd>
             <p>
               This backend type expects a non-blocking character device as
-              input. The only accepted paths are /dev/random and /dev/hwrng.
-              The file name is specified as contents of the
+              input. The file name is specified as contents of the
               <code>backend</code> element. When no file name is specified
-              the hypervisor default is used.
+              the hypervisor default is used. For qemu, the default is
+              /dev/random
             </p>
           </dd>
           <dt><code>egd</code></dt>

docs/schemas/domaincommon.rng

@@ -4693,8 +4693,7 @@
             <value>random</value>
           </attribute>
           <choice>
-            <value>/dev/random</value>
-            <value>/dev/hwrng</value>
+            <ref name='absFilePath'/>
             <empty/>
           </choice>
         </group>

src/conf/domain_conf.c

@@ -11497,14 +11497,6 @@ virDomainRNGDefParseXML(xmlNodePtr node,
     switch ((virDomainRNGBackend) def->backend) {
     case VIR_DOMAIN_RNG_BACKEND_RANDOM:
         def->source.file = virXPathString("string(./backend)", ctxt);
-        if (def->source.file &&
-            STRNEQ(def->source.file, "/dev/random") &&
-            STRNEQ(def->source.file, "/dev/hwrng")) {
-            virReportError(VIR_ERR_XML_ERROR,
-                           _("file '%s' is not a supported random source"),
-                           def->source.file);
-            goto error;
-        }
         break;
 
     case VIR_DOMAIN_RNG_BACKEND_EGD:

tests/qemuxml2argvdata/qemuxml2argv-virtio-rng-random.args

@@ -18,6 +18,6 @@ QEMU_AUDIO_DRV=none \
 -boot c \
 -usb \
 -device virtio-balloon-pci,id=balloon0,bus=pci.0,addr=0x3 \
--object rng-random,id=objrng0,filename=/dev/hwrng \
+-object rng-random,id=objrng0,filename=/dev/urandom \
 -device virtio-rng-pci,rng=objrng0,id=rng0,max-bytes=123,period=1234,bus=pci.0,\
 addr=0x4

tests/qemuxml2argvdata/qemuxml2argv-virtio-rng-random.xml

@@ -21,7 +21,7 @@
     <memballoon model='virtio'/>
     <rng model='virtio'>
       <rate bytes='123' period='1234'/>
-      <backend model='random'>/dev/hwrng</backend>
+      <backend model='random'>/dev/urandom</backend>
     </rng>
   </devices>
 </domain>

tests/qemuxml2xmloutdata/qemuxml2xmlout-virtio-rng-random.xml

@@ -25,7 +25,7 @@
     </memballoon>
     <rng model='virtio'>
       <rate bytes='123' period='1234'/>
-      <backend model='random'>/dev/hwrng</backend>
+      <backend model='random'>/dev/urandom</backend>
       <address type='pci' domain='0x0000' bus='0x00' slot='0x04' function='0x0'/>
     </rng>
   </devices>