vdpa_sim: make IOTLB entries limit configurable

Some devices may require a higher limit for the number of IOTLB
entries, so let's make it configurable through a module parameter.

By default, it's initialized with the current limit (2048).

Suggested-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Link: https://lore.kernel.org/r/20201215144256.155342-5-sgarzare@redhat.com


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