common: add requirement for unlimited virtual memory

Some tests fail if overcommit is disabled and virtual
memory is limited due to large memory requirements.
Such tests can require both overcommit enabled and
unlimited virtual memory.
Merge pull request #87 from plebioda/fix-overcommit-memory-issue

common: add requirement for overcommit memory in unittests
vmem: destroy pool_id in pool_destroy()

Ref: pmem/issues#20