Introduce wrapper functions to access phys_ram_dirty.

Adds wrapper functions to prevent direct access to the phys_ram_dirty bitmap.

Signed-off-by: Yoshiaki Tamura <tamura.yoshiaki@lab.ntt.co.jp>
Signed-off-by: OHMURA Kei <ohmura.kei@lab.ntt.co.jp>
Reviewed-by: Avi Kivity <avi@redhat.com>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
