Introduced VG-aware disk space checks

Implemented _CheckNodesFreeDiskPerVG and _ComputeDiskSizePerVG functions
for VG-aware iterations.

Signed-off-by: Dmitry Chernyak <dmi.chernyak@gmail.com>
[iustin@google.com: some style fixes (long lines, etc.)]
Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>