In plain to drbd conversion, rename LVs back on failure

Currently, if converting an instance from plain to drbd fails after
renaming the original LVs, the instance is left in an inconsistent
state. This commit tries to undo the renaming if a failure occurs
on assembling a DRBD device, e.g., when device minor numbers are
exhausted. (Issue 229)

Signed-off-by: Klaus Aehlig <aehlig@google.com>
Reviewed-by: Thomas Thrainer <thomasth@google.com>