Move the relocate.setupRelocation code into XendDomain, removing the mutual

Move the relocate.setupRelocation code into XendDomain, removing the mutual
dependency between those two classes.

Split the XendDomain constructor into two pieces so that XendDomainInfo can
call XendDomain.instance even during the initial startup of XendDomain.

These two together mean that there is no need for the horrendous hack using
XendRoot.get_component to pass the singleton instance of XendDomain around.
Both XendRoot.get_component and XendRoot.add_component can go.

Signed-off-by: Ewan Mellor <ewan@xensource.com>