hammer2_open_devvp
int hammer2_open_devvp(const hammer2_devvp_list_t *devvpl, int ronly);
error = hammer2_open_devvp(&devvpl, ronly);
int hammer2_open_devvp(const hammer2_devvp_list_t *devvpl, int ronly);
error = hammer2_open_devvp(&devvpl, ronly);