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
I would not stress too much about it. Brief code inspection shows that people use both

$ git grep '1024 \* 1024' | wc -l
50
$ git grep '1024 \*\*' | wc -l
12

... in _my_ youth we used to write 'x << 30'.