bsd-user: save the path to the qemu emulator

Save the path to the qemu emulator. This will be used later when we have
a more complete implementation of exec.

Signed-off-by: Stacey Son <sson@FreeBSD.org>
Signed-off-by: Warner Losh <imp@bsdimp.com>
Acked-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Kyle Evans <kevans@FreeBSD.org>