Enable virtio for tests

Change-Id: I1d0e546f6ea837928ba426e4690007ef2649f68e

BUG=10102
TEST=test KVM with -net virtio and ensure it boots and can copy files in etc

Review URL: http://codereview.chromium.org/5543006

image_to_vm.sh

@@ -284,7 +284,7 @@ fi
 
 if [ "${FLAGS_format}" == "qemu" ]; then
   echo "If you have qemu-kvm installed, you can start the image by:"
-  echo "sudo kvm -m ${FLAGS_mem} -vga std -pidfile /tmp/kvm.pid -net nic,model=e1000 " \
+  echo "sudo kvm -m ${FLAGS_mem} -vga std -pidfile /tmp/kvm.pid -net nic,model=virtio " \
        "-net user,hostfwd=tcp::9222-:22 \\"
   echo "        -hda ${FLAGS_to}/${DEFAULT_QEMU_IMAGE}"
 fi

lib/cros_vm_lib.sh

@@ -55,7 +55,7 @@ function start_kvm() {
       -vga std \
       -pidfile "${KVM_PID_FILE}" \
       -daemonize \
-      -net nic,model=e1000 \
+      -net nic,model=virtio \
       ${nographics} \
       ${snapshot} \
       -net user,hostfwd=tcp::${FLAGS_ssh_port}-:22 \