monitor: use DIV_ROUND_UP

I used the clang-tidy qemu-round check to generate the fix:
https://github.com/elmarco/clang-tools-extra

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Acked-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Richard Henderson <rth@twiddle.net>
