Apply by doing:
        cd /usr/src
        patch -p0 < 007_vio.patch

Then build and install a new kernel.

Index: sys/dev/pci/if_vio.c
===================================================================
RCS file: /cvs/src/sys/dev/pci/if_vio.c,v
retrieving revision 1.9
retrieving revision 1.9.2.2
diff -u -p -r1.9 -r1.9.2.2
--- sys/dev/pci/if_vio.c	5 Dec 2012 23:20:20 -0000	1.9
+++ sys/dev/pci/if_vio.c	30 May 2013 21:52:12 -0000	1.9.2.2
@@ -232,11 +232,13 @@ struct vio_softc {
 	((sc)->sc_hdr_size == sizeof(struct virtio_net_hdr))
 
 #define VIRTIO_NET_TX_MAXNSEGS		16 /* for larger chains, defrag */
-#define VIRTIO_NET_CTRL_MAC_MAXENTRIES	64 /* for more entries, use ALLMULTI */
+#define VIRTIO_NET_CTRL_MAC_MC_ENTRIES	64 /* for more entries, use ALLMULTI */
+#define VIRTIO_NET_CTRL_MAC_UC_ENTRIES	 1 /* one entry for own unicast addr */
 
 #define VIO_CTRL_MAC_INFO_SIZE 					\
 	(2*sizeof(struct virtio_net_ctrl_mac_tbl) + 		\
-	 (VIRTIO_NET_CTRL_MAC_MAXENTRIES + 1) * ETHER_ADDR_LEN)
+	 (VIRTIO_NET_CTRL_MAC_MC_ENTRIES + 			\
+	  VIRTIO_NET_CTRL_MAC_UC_ENTRIES) * ETHER_ADDR_LEN)
 
 /* cfattach interface functions */
 int	vio_match(struct device *, void *, void *);
@@ -391,9 +393,7 @@ vio_alloc_mem(struct vio_softc *sc)
 		allocsize += sizeof(struct virtio_net_ctrl_cmd) * 1;
 		allocsize += sizeof(struct virtio_net_ctrl_status) * 1;
 		allocsize += sizeof(struct virtio_net_ctrl_rx) * 1;
-		allocsize += sizeof(struct virtio_net_ctrl_mac_tbl)
-			+ sizeof(struct virtio_net_ctrl_mac_tbl)
-			+ ETHER_ADDR_LEN * VIRTIO_NET_CTRL_MAC_MAXENTRIES;
+		allocsize += VIO_CTRL_MAC_INFO_SIZE;
 	}
 	sc->sc_dma_size = allocsize;
 
@@ -413,7 +413,8 @@ vio_alloc_mem(struct vio_softc *sc)
 		sc->sc_ctrl_rx = (void*)(kva + offset);
 		offset += sizeof(*sc->sc_ctrl_rx);
 		sc->sc_ctrl_mac_tbl_uc = (void*)(kva + offset);
-		offset += sizeof(*sc->sc_ctrl_mac_tbl_uc);
+		offset += sizeof(*sc->sc_ctrl_mac_tbl_uc) +
+		    ETHER_ADDR_LEN * VIRTIO_NET_CTRL_MAC_UC_ENTRIES;
 		sc->sc_ctrl_mac_tbl_mc = (void*)(kva + offset);
 	}
 
@@ -742,6 +743,10 @@ again:
 			break;
 		}
 		IFQ_DEQUEUE(&ifp->if_snd, m);
+		if (m != sc->sc_tx_mbufs[slot]) {
+			m_freem(m);
+			m = sc->sc_tx_mbufs[slot];
+		}
 
 		hdr = &sc->sc_tx_hdrs[slot];
 		memset(hdr, 0, sc->sc_hdr_size);
@@ -1113,7 +1118,6 @@ vio_encap(struct vio_softc *sc, int slot
 		    r);
 		return ENOBUFS;
 	}
-	m_freem(m);
 	*mnew = m0;
 	return 0;
 }
@@ -1342,7 +1346,7 @@ vio_iff(struct vio_softc *sc)
 	}
 
 	if (ifp->if_flags & IFF_PROMISC || ac->ac_multirangecnt > 0 ||
-	    ac->ac_multicnt >= VIRTIO_NET_CTRL_MAC_MAXENTRIES) {
+	    ac->ac_multicnt >= VIRTIO_NET_CTRL_MAC_MC_ENTRIES) {
 		ifp->if_flags |= IFF_ALLMULTI;
 		if (ifp->if_flags & IFF_PROMISC)
 			promisc = 1;