LVM: remove old tags when adding new ones

This patch adds a small helper function to clear an LV's tags, and
calls it at SetInfo time. We need this to be able to correctly track
instance renames, once we will call SetInfo at such times.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>