Video playback on the Raspberry Pi is actually a bit of a tricky thing. The most common solution is to use OMXPlayer to play videos. It’s a command line program, but there are lots of flags and repeated calls can control playback. All in all, it works pretty dang well for most smallish projects.
As far as actually using Qt, QMediaPlayer, and etc. I don’t think you’ll be able to make it work right out of the box. However, I found a blog that discusses this issue and has some Raspberry Pi firmware that fixes QtMultimedia support on the Raspberry Pi.