bhyve: vtnet: fix locking on receive

The vsc_rx_ready and the RX virtqueue is protected by the rx_mtx lock.
However, pci_vtnet_ping_rxq() (currently called only once after each
device reset) accesses those without acquiring the lock.

Reviewed by: markj
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D20609