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.

BTW I've noticed a few places were we could improve our code:
* In SR.py we're setting the LVM_SYSTEM_DIR variable, we should be doing that in LVHDSR.py
* We set the LVM_SYSTEM_DIR variable in various places , while we should only be doing that on a single place.
Shall we address these issues in this ticket?

Signed-off-by: Thanos Makatos <thanos.makatos@citrix.com>