vdpa: Enable a user to add and delete a vdpa device

Add the ability to add and delete a vdpa device.

Examples:
Create a vdpa device of type network named "foo2" from
the management device vdpasim:

$ vdpa dev add mgmtdev vdpasim_net name foo2

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>
Reviewed-by: Jason Wang <jasowang@redhat.com>
Link: https://lore.kernel.org/r/20210105103203.82508-5-parav@nvidia.com
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>