virNetworkObjFindBy*: Return an reference to found object
    
    This patch turns both virNetworkObjFindByUUID() and
    virNetworkObjFindByName() to return an referenced object so that
    even if caller unlocks it, it's for sure that object won't
    disappear meanwhile. Especially if the object (in general) is
    locked and unlocked during the caller run.
    Moreover, this commit is nicely small, since the object unrefing
    can be done in virNetworkObjEndAPI().
    
    Signed-off-by: Michal Privoznik <mprivozn@redhat.com>