page-vary: Build migration_legacy_page_bits as common code

Use target_info->page_bits_init to implement
migration_legacy_page_bits.  Because this is the last
function within the file, rename page-vary-target.c
to page-vary-system.c and build once for system mode.

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-ID: <20260217095106.598486-7-richard.henderson@linaro.org>
[PMD: Update MAINTAINERS]
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>