qemu-check.exp: remove duplicate 'virt' keyword

Fixes 'make check' error introduced by commit 6f0c0eb9 ("qemu: Add
'-machine secure=on' to arguments").

Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>
Reviewed-by: Victor Chong <victor.chong@linaro.org>
Reviewed-by: Pascal Brand <pascal.brand@linaro.org>