virNetworkObjFindBy*: Return an reference to found object

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>