app: Create qemu session source file if its empty
app: Create qemu session source file if its empty

Instead of only creating qemu session source file if it doesn't exist,
also check if its empty and create it if thats the case.

This fixes a crash that can happen if Boxes crashed on a previous run
between the time the file is created and copied.

https://bugzilla.gnome.org/show_bug.cgi?id=730022

vbenes/gnome-boxes,vbenes/gnome-boxes