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

This reverts commit 060a23223163fab3656a323a0eabaaeaa6ea24be.
Revert "revert CA-114341 patches"

This reverts commit 5d1fe87240d43e2c31bce5dade4f6261d234b0f5.
CA-114341: increase lock retry count

The current value of lock retry is too small and results in VDI.activate
giving up trying to acquire the lock too early. Doubling this value
seems to rectify the problem.

Signed-off-by: Thanos Makatos <thanos.makatos@citrix.com>