openib/connect: re-enable xrc support in the openib btl

This commit updates the udcm cpc to support xrc. The steps followed by udcm
mimic those in the removed xoob cpc. This update has been tested with both XRC
and RC.

Mellanox, this is intended to go into 1.7.4. Please review carefully and let
me know if there are any issues.

cmr=v1.7.4:reviewer=miked