virtio: Use bus_dma for ring and indirect buffer allocations

[sarah.walker2_arm.com]

Summary

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.

Sponsored by: Arm Ltd

[jrtc27]

Do we still need the mb()s? I would expect bus_dmamap_sync to suffice.

[jrtc27]

virtio_blk calls bus_dma_tag_set_iommu on PowerPC; would that not be needed for these new tags?

[sarah.walker2_arm.com]

Remove superfluous mb() calls.

[andrew]

In D54959#1259073, @jrtc27 wrote:

virtio_blk calls bus_dma_tag_set_iommu on PowerPC; would that not be needed for these new tags?

It looks like that was added in D37891 by @cperciva. Based on the commit message we could add it here, but can't test.

[sarah.walker2_arm.com]

Add iommu tag calls for PowerPC.