hw/armv7m_nvic: Fix incorrect default for num-irqs property

Fix an incorrect default value for the num-irqs property (we were
attempting to override it from the default set by the parent class
but not succeeding, which meant that the lm3s6965evb model would
assert on startup attempting to wire up nonexistent irq lines).
Instead of trying to override the parent's Property array, we
define an instance_init function which runs after default setup
but before user property setting and can just fix up the default
value in the gic_state struct.

Reported-by: Peter Crosthwaite <peter.crosthwaite@petalogix.com>
Tested-by: Peter Crosthwaite <peter.crosthwaite@petalogix.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
