CA-128175: vhd-util check on ~TB sized disk taking long.

vhd-util check by default does a quadratic check on the BAT that takes a long time
(In the order of 10 mins) to finish. This inadvertently could block leaf
coalesce. Skipping of quadratic BAT check was introduced via CA-90801
for non-leaf coalesce case.

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

GitHub: closes #82 on xapi-project/sm