hw/intc/arm_gicv3: Move checking of redist-region-count to arm_gicv3_common_realize

hw/intc/arm_gicv3: Move checking of redist-region-count to arm_gicv3_common_realize

The GICv3 devices have an array property redist-region-count.
Currently we check this for errors (bad values) in
gicv3_init_irqs_and_mmio(), just before we use it.  Move this error
checking to the arm_gicv3_common_realize() function, where we
sanity-check all of the other base-class properties. (This will
always be before gicv3_init_irqs_and_mmio() is called, because
that function is called in the subclass realize methods, after
they have called the parent-class realize.)

The motivation for this refactor is:
 * we would like to use the redist_region_count[] values in
   arm_gicv3_common_realize() in a subsequent patch, so we need
   to have already done the sanity-checking first
 * this removes the only use of the Error** argument to
   gicv3_init_irqs_and_mmio(), so we can remove some error-handling
   boilerplate

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