uml: Use virDomainObjListFindBy{UUID|ID}Ref

uml: Use virDomainObjListFindBy{UUID|ID}Ref

For umlDomObjFromDomainLocked and umlDomainLookupByID 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. This
means for some consumers we need to relock the @dom after a
virDomainObjListRemove, but before calling virDomainObjEndAPI.

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: Daniel P. Berrangé <berrange@redhat.com>