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=1785091Signed-off-by: NHan Han <hhan@redhat.com>
Reviewed-by: NMichal Privoznik <mprivozn@redhat.com>