Implement 'get_domain_zero_policy'

This allows xapi to query the configured domain zero memory policy,
which can be configured via the squeezed configuration file.

This change must be merged before djs55/xcp-idl#10

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Merge pull request #16 from djs55/domain-zero-policy

Implement 'get_domain_zero_policy'