Introduce use of a virDomainDefPtr in the domain stats &
peek APIs to simplify introduction of ACL security checks.
The virDomainPtr cannot be safely used, since the app
may have supplied mis-matching name/uuid/id fields. eg
the name points to domain X, while the uuid points to
domain Y. Resolving the virDomainPtr to a virDomainDefPtr
ensures a consistent name/uuid/id set.
Signed-off-by: NDaniel P. Berrange <berrange@redhat.com>