vdpa: Add support for returning device configuration information

Add netlink attribute to store the negotiated features. This can be used
by userspace to get the current state of the vdpa instance.

Examples:

$ vdpa dev config show vdpa-a
vdpa-a: mac 00:00:00:00:88:88 link up link_announce false max_vq_pairs 16 mtu 1500
  negotiated_features CSUM GUEST_CSUM MTU MAC HOST_TSO4 HOST_TSO6 STATUS \
  CTRL_VQ MQ CTRL_MAC_ADDR VERSION_1 ACCESS_PLATFORM

$ vdpa -j dev config show vdpa-a
{"config":{"vdpa-a":{"mac":"00:00:00:00:88:88","link ":"up","link_announce":false, \
 "max_vq_pairs":16,"mtu":1500,"negotiated_features":["CSUM","GUEST_CSUM","MTU","MAC", \
 "HOST_TSO4","HOST_TSO6","STATUS","CTRL_VQ","MQ","CTRL_MAC_ADDR","VERSION_1", \
 "ACCESS_PLATFORM"]}}}

$ vdpa -jp dev config show vdpa-a
{
    "config": {
        "vdpa-a": {
            "mac": "00:00:00:00:88:88",
            "link ": "up",
            "link_announce ": false,
            "max_vq_pairs": 16,
            "mtu": 1500,
            "negotiated_features": [
"CSUM","GUEST_CSUM","MTU","MAC","HOST_TSO4","HOST_TSO6","STATUS","CTRL_VQ","MQ", \
"CTRL_MAC_ADDR","VERSION_1","ACCESS_PLATFORM"
]
        }
    }
}

Signed-off-by: Eli Cohen <elic@nvidia.com>
Link: https://lore.kernel.org/r/20220105114646.577224-9-elic@nvidia.com
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>