memory: allow domain zero memory policy to be queried

Now that squeezed supports two different domain zero memory policies,
it is useful for xapi to be able to find out which policy is in effect
so it can populate the domain 0 VM fields correctly.

The two policies are:

 Fixed_size x
    -- domain zero will always occupy x bytes: this is the default
       XenServer iso configuration
 Auto_balloon (low, high)
    -- domain zero will be allowed to balloon between low and high:
       this is a likely configuration for a development environment

Signed-off-by: David Scott <dave.scott@eu.citrix.com>