vDPA/ifcvf: implement management netlink framework for ifcvf

This commit implements the management netlink framework for ifcvf,
including register and add / remove a device

It works with iproute2:
[root@localhost lszhu]# vdpa mgmtdev show -jp
{
    "mgmtdev": {
        "pci/0000:01:00.5": {
            "supported_classes": [ "net" ]
        },
        "pci/0000:01:00.6": {
            "supported_classes": [ "net" ]
        }
    }
}

[root@localhost lszhu]# vdpa dev add mgmtdev pci/0000:01:00.5 name vdpa0
[root@localhost lszhu]# vdpa dev add mgmtdev pci/0000:01:00.6 name vdpa1

Signed-off-by: Zhu Lingshan <lingshan.zhu@intel.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Link: https://lore.kernel.org/r/20210812032454.24486-3-lingshan.zhu@intel.com
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>