accel/tcg: Use TLB_FORCE_SLOW not TLB_MMIO for system plugins

In b79a9b6e5b, we stopped using TLB_MMIO for user-only plugins,
but erroneously thought we were already doing the same for system.
Do that now.

Cc: qemu-stable@nongnu.org
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
(cherry picked from commit cab8663761943141f8fd75dddc99255fe2cb45c4)
(Mjt: b79a9b6e5b is commit v11.0.2-61-g497273cecfb1 in 11.0.x)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>