vdpasim: customize allocation size

Allow individual simulator to customize the allocation size.

Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Acked-by: Eugenio Pérez <eperezma@redhat.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>
Message-Id: <20221223055548.27810-3-jasowang@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>