device-qemu-*: set a default resolution for X11

Qemu by default starts a GTK window in VGA resolution 640x480. The content
adapts to this resolution, thus X11 shows up in resolution 640x480 as well.

This resolution causes some handling issues. In Xfce4 the shutdown button of
the menu isn't reachable. Even worse in MATE, where the "Apply" button for
changing the resolution is out of the screen, making it difficult to change
the resolution.

The default resolution for X11 can be set by a config file. The Qemu GTK
window resolution remains independently at default 640x480. Setting the X11
resolution to default 800x600 mitigates the issue: Most of the screen content
becomes accessible while font size and button sizes are still large enough.
From this point the user can adjust the resolutions of the Qemu GTK window
and X11 content as desired.