Revert changes made for CA-122205

Roll back fixes that ignore journal unavailability exceptions.
Issue being fixed under CA-129800 by correcting the original changes
made for CP-5818. Commits being reverted are:

* 2171f7d: "CA-122205: More race conditions."
* 4e92794: "CA-122205: Continuation to the previous commits"
* 84dcbf4: "CA-122205: Addressing build errors, also backing out blktap2 changes"
* 68286d1: "CA-122205: Resolve race between VDI.activate and GC."

Signed-off-by: Chandrika Srinivasan <chandrika.srinivasan@citrix.com>
Reviewed-by: Vineeth Thampi Raveendran <vineeth.thampi@citrix.com>

GitHub: closes #96 on xapi-project/sm