Adding functionality to libvirt that will allow querying the interface
for the availability of switchdev Offloading NIC capabilities.

The switchdev mode was introduced in kernel 4.8, the iproute2-devlink
command to retrieve the switchdev NIC feature with command example:

    devlink dev eswitch show pci/0000:03:00.0

This feature is needed for Openstack so we can do a scheduling decision
if the NIC is in Hardware Offload (switchdev) or regular SR-IOV (legacy) mode.
And select the appropriate hypervisors with the requested capability see [1].

[1] - https://specs.openstack.org/openstack/nova-specs/specs/pike/approved/enable-sriov-nic-features.htmlReviewed-by: NLaine Stump <laine@laine.org>
Reviewed-by: NJohn Ferlan <jferlan@redhat.com>