hw/arm/virt: Abstract out calculation of redistributor region capacity

hw/arm/virt: Abstract out calculation of redistributor region capacity

In several places in virt.c we calculate the number of redistributors that
fit in a region of our memory map, which is the size of the region
divided by the size of a single redistributor frame. For GICv4, the
redistributor frame is a different size from that for GICv3. Abstract
out the calculation of redistributor region capacity so that we have
one place we need to change to handle GICv4 rather than several.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20220408141550.1271295-41-peter.maydell@linaro.org