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>

Also amended a comment of a tested method.
The tests are for methods introduced in commit 63c73073 (LVM disk creation
uses dedicated PVs).

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