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

This reverts commit e4a239c3dcc901d63ffb71644e532c8d050b84aa. The commit
is reverted because it has apparently exposed an existing bug. Will
reapply the commit once this is confirmed and the bug fixed.

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

Github: closes #80 on xapi-project/sm