spice-display: Don't set 'only-downscale' prop on display

Setting this only makes the display very tiny on hidpi screens when no
autoresolution is in effect (e.g grub screen or guests with no
qxl/agent). Lets not ask spice to only downscale but rather always scale
the display to fit in its window.

https://bugzilla.gnome.org/show_bug.cgi?id=733978