display: set min value for desktop-{width,height} props as MIN_DISPLAY_{WIDTH,HEIGHT}
Otherwise we can have warnings when resizing the virt-viewer window to
the smallest possible size, like:
(virt-viewer:11187): GLib-GObject-WARNING **: value "50" of type `gint'
is invalid or out of range for property `desktop-height' of type `gint'
Related: rhbz#1296878