lib: a vrf is searched first by its name, than its vrf id
Because at startup, remote daemons attempt to create default VRF,
the VRF_ID may be set to unknown. In that case, an event will be
triggered later by zebra to inform remote daemon that the vrf id of that
VRF has changed to valid value. In that case, two instances of default
VRF must not be created. By looking first at vrf name, this avoids
having two instances.