test: enable virtio-rng device for QEMU guests

If rngd is included in the host initrd, QEMU guests need at least one source of
entropy otherwise rngd will refuse to start. Hence this patch enables the
virtio RNG device in QEMU guests (exposed as a HW RNG device available at
/dev/hwrng).

As a safety measure, the patch limits the data sent to the guest to 1KB per
second in order to not let the guest starve the host entropy.

test/test-functions

@@ -452,6 +452,7 @@ run_qemu() {
         -nographic
         -kernel "$KERNEL_BIN"
         -drive "format=raw,cache=unsafe,file=$image"
+        -device "virtio-rng-pci,max-bytes=1024,period=1000"
     )
 
     if [[ -n "${QEMU_OPTIONS:=}" ]]; then