Enable virtio rng device for net tests on qemu/kvm.

Before:
  root@(none):/host# dmesg | egrep -i 'virtio|random|rnd|rng'
  [    0.000000] random: get_random_bytes called from start_kernel+0x94/0x445 with crng_init=0
  [    1.068452] random: fast init done
  [    1.226737] random: all_tests.py: uninitialized urandom read (2500 bytes read)
  [    1.297999] random: all_tests.py: uninitialized urandom read (2500 bytes read)
  [    1.301358] random: all_tests.py: uninitialized urandom read (2500 bytes read)
  [   51.829187] urandom_read: 1 callbacks suppressed
  [   51.829189] random: all_tests.py: uninitialized urandom read (8 bytes read)
  [   51.833191] random: all_tests.py: uninitialized urandom read (8 bytes read)
  [   51.836113] random: all_tests.py: uninitialized urandom read (8 bytes read)
  [   78.285661] random: crng init done
  [   78.287299] random: 3 urandom warning(s) missed due to ratelimiting

After:
  root@(none):/host# dmesg | egrep -i 'virtio|random|rnd|rng'
  [    0.000000] random: get_random_bytes called from start_kernel+0x94/0x445 with crng_init=0
  [    1.018666] random: fast init done
  [    1.020632] random: crng init done

  root@(none):/host# lspci
  00:00.0 Host bridge: Intel Corporation 440FX - 82441FX PMC [Natoma] (rev 02)
  00:01.0 ISA bridge: Intel Corporation 82371SB PIIX3 ISA [Natoma/Triton II]
  00:01.1 IDE interface: Intel Corporation 82371SB PIIX3 IDE [Natoma/Triton II]
  00:01.3 Bridge: Intel Corporation 82371AB/EB/MB PIIX4 ACPI (rev 03)
  00:02.0 Unclassified device [00ff]: Red Hat, Inc Virtio RNG
  00:03.0 Unclassified device [0002]: Red Hat, Inc Virtio filesystem
  00:04.0 SCSI storage controller: Red Hat, Inc Virtio block device

  (note the new Virtio RNG device)

Change-Id: Ifea0c261af1209f9307adb150cf32f7bc92aba74
diff --git a/net/test/run_net_test.sh b/net/test/run_net_test.sh
index 8249767..54e64a4 100755
--- a/net/test/run_net_test.sh
+++ b/net/test/run_net_test.sh
@@ -300,6 +300,7 @@
     -kernel $KERNEL_BINARY \
     -no-user-config -nodefaults -no-reboot -display none \
     -machine pc,accel=kvm -cpu host -smp 4,sockets=4,cores=1,threads=1 \
+    -device virtio-rng-pci \
     -chardev file,id=exitcode,path=exitcode \
     -device isa-serial,chardev=exitcode \
     -fsdev local,security_model=mapped-xattr,id=fsdev0,fmode=0644,dmode=0755,path=$SCRIPT_DIR \