gnt-node volumes: Fix instance names

Commit 84d7e26b changed “objects.Instance.MapLVsByN” to not just return
the LV name, but to include the volume group name (e.g.
“xenvg/d67e8700….disk0_data”). This in turn broke the mapping of volume
names in LUNodeQueryvols, stopping instance names from displayed in
“gnt-node volumes”.

This patch fixes the issue and does some cleanup.

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