Add support in SR-IOV mechanism driver for VNIC type
"accelerator-direct".
This VNIC type refers to any device that supports hardware
acceleration of any kind and is provided by Cyborg [1].
NOTE: "accelerator-direct-physical" is not supported yet.
[1]https://wiki.openstack.org/wiki/Cyborg
Change-Id: I529e6a2a445a140dca7686976efeefcd13d333f0
Closes-Bug: #1909100