CA-117628: allow LVM metadata updates when running through plug-in

This patch uses the LVM configuration file that allows metadata updates when
running LVM commands on the master through the plug-in.

Signed-off-by: Thanos Makatos <thanos.makatos@citrix.com>
Reviewed-by: Germano Percossi <germano.percossi@citrix.com>
CA-117628: don't ignore result of VHD footer zeroing errors

Although this commit is not required for fixing the issue, it is still
important as we were previously ignoring the result of it and detected the
actual error much later on.

Signed-off-by: Thanos Makatos <thanos.makatos@citrix.com>
Reviewed-by: Germano Percossi <germano.percossi@citrix.com>
Merge CA-117628

Signed-off-by: Germano Percossi <germano.percossi@citrix.com>

GitHub: closes #93 on xapi-project/sm