vdpa_sim: add struct vdpasim_dev_attr for device attributes

vdpa_sim: add struct vdpasim_dev_attr for device attributes

vdpasim_dev_attr will contain device specific attributes. We starting
moving the number of virtqueues (i.e. nvqs) to vdpasim_dev_attr.

vdpasim_create() creates a new vDPA simulator following the device
attributes defined in the vdpasim_dev_attr parameter.

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/20201215144256.155342-7-sgarzare@redhat.com
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>