EA-1158/CP-5958: Replace assert with ASSERT

Calls to assert() kills tapdisk without leaving a clue. Using ASSERT
macro first logs and then kills tapdisk. The change is performed only
on the assert calls introduced by EA-1158.

Signed-off-by: Vineeth Thampi Raveendran <vineetht@citrix.com>
Reviewed-by: Thanos Makatos <thanos.makatos@citrix.com>

Github closes: #52 on xapi-projects/blktap