monitor: clear dnet_vm_stat at dnet_get_vm_stat.
monitor: added 'error' and 'string_error' to procfs statistics: vm, io and proc.
Pytests: if procfs statistics: vm, io or stat; contain nonzero error - do not asserts internal statistics.
Merge pull request #535 from shaitan/master

Fixed garbage at unitialized vm statistics if procfs and sysctl are not available.