if_vtnet: let vtnet_rx_vq_intr() and vtnet_rxq_tq_intr() share code

Since the two functions are similar, introduce a common function
(vtnet_rx_vq_process()) to share common code.
This also improves locking, by ensuring vrxs_rescheduled is accessed
under the RXQ lock, and taskqueue_enqueue() is not called under the
lock (therefore avoiding a spurious duplicate lock warning).

Reported by: jrtc27
MFC after: 2 weeks