qemu.tests: Correct the post operate cmd in win_virtio_update

Correct the way we get post operate cmd in win_virtio_update.

Signed-off-by: Yiqiao Pu <ypu@redhat.com>
qemu.tests: Update the virtio cdrom path in win_virtio_update

Update the virtio cdrom path so it can handle the relative path we
normally used for the data dir iso directory.

Signed-off-by: Yiqiao Pu <ypu@redhat.com>