arm v8M: Forcibly clear negative-priority exceptions on deactivate

arm v8M: Forcibly clear negative-priority exceptions on deactivate

To prevent execution priority remaining negative if the guest
returns from an NMI or HardFault with a corrupted IPSR, the
v8M interrupt deactivation process forces the HardFault and NMI
to inactive based on the current raw execution priority,
even if the interrupt the guest is trying to deactivate
is something else. In the pseudocode this is done in the
Deactivate() function.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20190617175317.27557-3-peter.maydell@linaro.org