memory: track DIRTY_MEMORY_CODE in mr->dirty_log_mask

DIRTY_MEMORY_CODE is only needed for TCG.  By adding it directly to
mr->dirty_log_mask, we avoid testing for TCG everywhere a region is
checked for the enabled/disabled state of dirty logging.

Reviewed-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
