vdpa_sim: make vdpasim->buffer size configurable

Allow each device to specify the size of the buffer allocated
in vdpa_sim.

Acked-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Link: https://lore.kernel.org/r/20201215144256.155342-17-sgarzare@redhat.com


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