make AXI stream stall statistics available to users

[eine]

In #557, the possibility to configure random stalls was added to AXI Stream VCs. In the same PR, a statistics feature was added to the sources for testing. As commented in #557 (comment), it would be useful to make these statistics available for a regular usage of the VCs (a kind of monitor). This issue is a remainder, so that we don't forget that we have the feature in the codebase already.