block/dirty-bitmap: switch _next_dirty_area and _next_zero to int64_t

We are going to introduce bdrv_dirty_bitmap_next_dirty so that same
variable may be used to store its return value and to be its parameter,
so it would int64_t.

Similarly, we are going to refactor hbitmap_next_dirty_area to use
hbitmap_next_dirty together with hbitmap_next_zero, therefore we want
hbitmap_next_zero parameter type to be int64_t too.

So, for convenience update all parameters of *_next_zero and
*_next_dirty_area to be int64_t.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: John Snow <jsnow@redhat.com>
Message-id: 20200205112041.6003-6-vsementsov@virtuozzo.com
Signed-off-by: John Snow <jsnow@redhat.com>