qemu: Implement builtin rng backend

Since v4.2-rc0, QEMU introduced a builtin rng backend that uses
getrandom() syscall to generate random. Add it to libvirt with the
backend model 'builtin'.

https://bugzilla.redhat.com/show_bug.cgi?id=1785091

Signed-off-by: Han Han <hhan@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>

tests/domaincapsdata/qemu_4.2.0.s390x.xml

@@ -190,6 +190,7 @@
       <enum name='backendModel'>
         <value>random</value>
         <value>egd</value>
+        <value>builtin</value>
       </enum>
     </rng>
   </devices>