virtio_net: Use bus_dma for command/ack buffers

While the majority of virtio platforms will be fully coherent, some may
require cache maintenance or other specific device memory handling (eg for
secure partitioning). Using bus_dma allows for these usecases.

The virtio buffers are marked as coherent; this should ensure that sync
calls are no-ops in the common cases.

Reviewed by: andrew
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D55564