nvic: Clear the vector arrays and prigroup on reset

nvic: Clear the vector arrays and prigroup on reset

Reset for devices does not include an automatic clear of the
device state (unlike CPU state, where most of the state
structure is cleared to zero). Add some missing initialization
of NVIC state that meant that the device was left in the wrong
state if the guest did a warm reset.

(In particular, since we were resetting the computed state like
s->exception_prio but not all the state it was computed
from like s->vectors[x].active, the NVIC wound up in an
inconsistent state that could later trigger assertion failures.)

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-id: 1506092407-26985-2-git-send-email-peter.maydell@linaro.org