vdpa_sim_net: Add support for user supported devices

vdpa_sim_net: Add support for user supported devices

Enable user to create vdpasim net simulate devices.

Show vdpa management device that supports creating, deleting vdpa devices.

$ vdpa mgmtdev show
vdpasim_net:
  supported_classes
    net

$ vdpa mgmtdev show -jp
{
    "show": {
        "vdpasim_net": {
            "supported_classes": {
              "net"
        }
    }
}

Create a vdpa device of type networking named as "foo2" from
the management device vdpasim:

$ vdpa dev add mgmtdev vdpasim_net name foo2

Show the newly created vdpa device by its name:
$ vdpa dev show foo2
foo2: type network mgmtdev vdpasim_net vendor_id 0 max_vqs 2 max_vq_size 256

$ vdpa dev show foo2 -jp
{
    "dev": {
        "foo2": {
            "type": "network",
            "mgmtdev": "vdpasim_net",
            "vendor_id": 0,
            "max_vqs": 2,
            "max_vq_size": 256
        }
    }
}

Delete the vdpa device after its use:
$ vdpa dev del foo2

Signed-off-by: Parav Pandit <parav@nvidia.com>
Reviewed-by: Eli Cohen <elic@nvidia.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Link: https://lore.kernel.org/r/20210105103203.82508-7-parav@nvidia.com
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>