BUGFIX: PCI-3781 nova-compute service is running vgs multiple times

- refactoring part of code to get all needed values in one call instead of
  consequenti multiple call of the same expensive external command
- optimalizing calls of new methods and functions little bit
Ahh,
 'x << 30' - I have to calm myself and remove tears from my eyes.  Thank you for remembering good old times.