CA-122205: More race conditions.

dmsetup table LV called from _checkActive could error out
if GC running in parallel has removed the LV.

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

GitHub: closes #95 on xapi-project/sm