vdpa_sim_blk: add support for vdpa management tool

vdpa_sim_blk: add support for vdpa management tool

Enable the user to create vDPA block simulator devices using the
vdpa management tool:

    # Show vDPA supported devices
    $ vdpa mgmtdev show
    vdpasim_blk:
      supported_classes block

    # Create a vDPA block device named as 'blk0' from the management
    # device vdpasim:
    $ vdpa dev add mgmtdev vdpasim_blk name blk0

    # Show the info of the 'blk0' device just created
    $ vdpa dev show blk0 -jp
    {
        "dev": {
            "blk0": {
                "type": "block",
                "mgmtdev": "vdpasim_blk",
                "vendor_id": 0,
                "max_vqs": 1,
                "max_vq_size": 256
            }
        }
    }

    # Delete the vDPA device after its use
    $ vdpa dev del blk0

Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Link: https://lore.kernel.org/r/20210315163450.254396-15-sgarzare@redhat.com
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>