This commit fixes a violation of the TLM2.0 protocol as well as a
bug regarding back-pressure:
- In the BEGIN_REQ phase, transaction objects are required to set
their response status to TLM_INCOMPLETE_RESPONSE. This was not
the case in the packet2payload function that converts gem5 packets
to TLM2.0 generic payloads.
- When the target applies back-pressure to the initiator, an assert
condition was triggered as soon as the response is retried. The
cause of this was an unintentional nullptr-access into a map.