For using offline maps in QML(Qt) you need to use OSM plugin. This plugin gives you posibility to take map tiles from your local directory. Names of tile files have to be format:
osm_100-<l|h>-<map_id>-<z>-<x>-<y>.<extension>
Where can we get tiles? You can download tiles by SAS.Planet
And use this app for convert folder with tiles to right format of OSM plugin.
For using run:
python main.py <tiles_dir> <map_id> <l|h>