libqmi-glib: allow qmux connection over unix domain socket

Hello,

I'm currently working on a research project for my university. Its goal is to connect libqmi with baseband chips in modern smartphones. For this goal, an application in-between is required. During its development, I explored multiple ways of interfacing with libqmi.

The emulation of pseudo-character device files in user space turned out rather tricky. In contrast, the implementation of Unix Domain Socket is simple and reliable. Thanks to it, I was able to establish a connection between libqmi and the baseband chip of a smartphone.

I created this merge request so other projects can profit from the simple access to libqmi. Let me know if parts of the merge request can be improved.

Best wishes
Lukas