migration/savevm.c: migrate non-default page size

migration/savevm.c: migrate non-default page size

Add a subsection to vmstate_configuration which is present
only if the guest is using a target page size which is
different from the default. This allows us to helpfully
diagnose attempts to migrate between machines which
are using different target page sizes.

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