armv7m_nvic: Fix m-security subsection name

The vmstate save/load code insists that subsections of a VMState must
have names which include their parent VMState's name as a leading
substring.  Unfortunately it neither documents this nor checks it on
device init or state save, but instead fails state load with a
confusing error message ("Missing section footer for armv7m_nvic").

Fix the name of the m-security subsection of the NVIC, so that
state save/load works correctly for the security-enabled NVIC.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20180727113854.20283-2-peter.maydell@linaro.org