GTK display does not support host scale factor

In the GNOME desktop environment, for HiDPI displays there is support to upscale everything.

This can be set in "System Settings -> Displays -> Scale".

I believe this affects GDK in the same way as setting the "GDK_SCALE" environment variable does.

When launching `qemu-system-x86_64 ... -display gtk`, this scale factor seems to get lost; the result is that the host window is upscaled and doubled in size, while the guest appears only in the bottom left corner of the UI.