openvz: Use virDomainObjListFindBy{UUID|ID}Ref

openvz: Use virDomainObjListFindBy{UUID|ID}Ref

For openvzDomObjFromDomainLocked and openvzDomainLookupByID
let's return a locked and referenced @vm object so that callers
can then use the common and more consistent virDomainObjEndAPI
in order to handle cleanup rather than needing to know that the
returned object is locked and calling virObjectUnlock.

The LookupByName already returns the ref counted and locked object,
so this will make things more consistent.

Signed-off-by: John Ferlan <jferlan@redhat.com>
Reviewed-by: Jim Fehlig <jfehlig@suse.com>