vdpa/mlx5: Enable user to add/delete vdpa device

Allow to control vdpa device creation and destruction using the vdpa
management tool.

Examples:
1. List the management devices
$ vdpa mgmtdev show
pci/0000:3b:00.1:
  supported_classes net

2. Create vdpa instance
$ vdpa dev add mgmtdev pci/0000:3b:00.1 name vdpa0

3. Show vdpa devices
$ vdpa dev show
vdpa0: type network mgmtdev pci/0000:3b:00.1 vendor_id 5555 max_vqs 16 \
max_vq_size 256

Signed-off-by: Eli Cohen <elic@nvidia.com>
Reviewed-by: Parav Pandit <parav@nvidia.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Link: https://lore.kernel.org/r/20210408091320.4600-1-elic@nvidia.com
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>