s390: fix softmmu compilation

guest_base must be used only in linux-user mode.

Signed-off-by: Laurent Vivier <laurent@vivier.eu>
Message-id: 1440757421-9674-1-git-send-email-laurent@vivier.eu
Reviewed-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>