vdpa: Introduce query of device config layout

Introduce a command to query a device config layout.

An example query of network vdpa device:

$ vdpa dev add name bar mgmtdev vdpasim_net

$ vdpa dev config show
bar: mac 00:35:09:19:48:05 link up link_announce false mtu 1500

$ vdpa dev config show -jp
{
    "config": {
        "bar": {
            "mac": "00:35:09:19:48:05",
            "link ": "up",
            "link_announce ": false,
            "mtu": 1500,
        }
    }
}

Signed-off-by: Parav Pandit <parav@nvidia.com>
Signed-off-by: Eli Cohen <elic@nvidia.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Link: https://lore.kernel.org/r/20211026175519.87795-3-parav@nvidia.com
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>