hw/arm/virt: Set minimum_page_bits to 12

hw/arm/virt: Set minimum_page_bits to 12

Since the virt board model will never create a CPU which is
pre-ARMv7, we know that our minimum page size is 4K and can
set minimum_page_bits accordingly, for improved performance.

Note that this is a migration compatibility break, so
we introduce it only for the virt-2.8 machine and onward;
virt-2.7 continues using the old 1K pages.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <rth@twiddle.net>