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

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."