hyperv/hn: add devctl_notify for VF_UP/DOWN events

Reviewed by:	sephe
Approved by:	sephe (mentor)
MFC after:	2 weeks
Sponsored by:	Microsoft
Differential Revision:	https://reviews.freebsd.org/D9102

sys/dev/hyperv/netvsc/if_hn.c

@@ -996,6 +996,9 @@ hn_set_vf(struct hn_softc *sc, struct ifnet *ifp, bool vf)
 		hn_resume_mgmt(sc);
 	}
 
+	devctl_notify("HYPERV_NIC_VF", if_name(hn_ifp),
+	    vf ? "VF_UP" : "VF_DOWN", NULL);
+
 	if (bootverbose)
 		if_printf(hn_ifp, "Data path is switched %s %s\n",
 		    vf ? "to" : "from", if_name(ifp));