Cluster verify: fix LV checks for split instances

When sharding by group, if a mirrored instance is split (primary and
secondary) between two groups, its volumes will not be properly checked:
the group of the primary will warn about a missing volume in the secondary,
and the group of the secondary about an unknown volume (in the secondary as
well).

To solve the "missing volumes" bit, we will detect this case and perform an
extra RPC verify call to these split secondaries (querying only for
NV_LVLIST), and introduce the results in the node images appropriately. We
do this detection early in ExpandNames/CheckPrereq, as to properly lock the
extra nodes.

As for the "unknown volumes" warning in the secondary, we update the volume
mapping with split instances before checking for orphaned volumes.

Finally, we mark nodes as "ghost" only if they really don't exist in the
cluster configuration, which avoid spurious "instance lives in ghost node"
warnings.

Signed-off-by: Adeodato Simo <dato@google.com>
Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>