qemu.test: Update the method for start vm in win_virtio_update

The old code will cause problems in post process for images as the
vms is set to empty.

Signed-off-by: Yiqiao Pu <ypu@redhat.com>
Merge pull request #103 from ypu/win_virtio_update

qemu.test: Update the method for start vm in win_virtio_update