utils.LvmExclusiveCheckNodePvs() returns more info

utils.LvmExclusiveCheckNodePvs() now returns the size of the smallest and
biggest PVs of a node. They will be used for cross-node consistency checks.

Signed-off-by: Bernardo Dal Seno <bdalseno@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>