Fix the fact that we can't bump RAM quotas above 2048M

(and acknowledge the reality that 2048M really isn't the rarest
thing to need these days)

Quoting from history:

"""
   invirt-xen-config (0.0.15) unstable; urgency=low

     * Set the max memory to a really high number so we can bump it for users
     * Pass the correct networking device to vif-invirtroute

    -- Evan Broder <broder@mit.edu>  Fri, 14 Nov 2008 00:03:47 -0500
"""

i,i "640K ought to be enough for anyone."