nixos/tests/vbox: Replace waitForWindow with xprop

We can't use waitForWindow here because it runs xwininfo as user root,
who in turn is not authorized to connect to the X server running as
alice.

So instead, we use xprop from user alice which should fix waiting for
the VirtualBox manager window.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>