Add pfil(9) hook to vtnet(4).

The patch could be simplier, using only the second chunk to
vtnet_rxq_eof(), that passes full mbufs to pfil(9). Packet
filter would m_free() them in case of returning PFIL_DROPPED.

However, we pretend to be a hardware driver, so we first try
to pass a memory buffer via PFIL_MEMPTR feature. This is mostly
done for debugging purposes, so that one can experiment in bhyve
with packet filters utilizing same features as a true driver.