vdpa_sim: add work_fn in vdpasim_dev_attr

Rename vdpasim_work() in vdpasim_net_work() and add it to
the vdpasim_dev_attr structure.

Co-developed-by: Max Gurtovoy <mgurtovoy@nvidia.com>
Signed-off-by: Max Gurtovoy <mgurtovoy@nvidia.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Link: https://lore.kernel.org/r/20201215144256.155342-10-sgarzare@redhat.com


Signed-off-by: Michael S. Tsirkin <mst@redhat.com>