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. In addition,
this patch introduces checks on the return value of a few functions.

Signed-off-by: Thanos Makatos <thanos.makatos@citrix.com>
Acked-by: Germano Percossi <germano.percossi@citrix.com>

GitHub: closes #70 on xapi-project/sm