Allow domain 0's memory policy to be configured via the config file

If 'manage-domain-zero = true' then we derive the domain 0 memory
configuration from the variables:

 domain-zero-dynamic-min = <byte value>
   -- where <byte value> is the minimum amount of memory we will
      balloon domain 0 down to, defaulting to 1 GiB

 domain-zero-dynamic-max = <auto|byte value>
   -- where 'auto' means use all available domain 0 memory (which
      may have been constrained from the xen commandline) and
      'byte value' means a specific amount. We default to 'auto'

Signed-off-by: David Scott <dave.scott@eu.citrix.com>