Revert "CA-114341: check return value of LVHD plugins"

This reverts commit 4bdb01aa761d73519c2e14410a3e63f291918524. We revert
the commit because it uncovers an existing bug (fixing it is not
trivial) and makes the BVT and BST fail, blocking the merge of
trunk-storage into trunk.

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

GitHub: closes #100 on xapi-project/sm