app-window: Explicitly unbind from Machine.status
app-window: Explicitly unbind from Machine.status

Although removing our only reference to the binding should take care of
destroying the binding, it doesn't seem to currently be happening. This
somehow leads to memory corruptions 'open_three_new_windows' test
scenario.

Lets explicitly unbind, at least until we figure out and get the actual
issue fixed. Likely a Vala bug.

vbenes/gnome-boxes,vbenes/gnome-boxes