qcow2-refcount: rename inc_refcounts() and make it public

This is needed for the following patch, which will introduce refcounts
checking for qcow2 bitmaps.

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: 20170628120530.31251-8-vsementsov@virtuozzo.com
[mreitz: s/inc_refcounts/qcow2_inc_refcounts_imrt/ in one more (new)
         place]
Signed-off-by: Max Reitz <mreitz@redhat.com>
