will be used in the next patch

https://jira.sw.ru/browse/PSBM-67502

note: actually we need only one value from inventory entry but I still
prefer general helper as we still need to read and allocate memory
for the whole structure

v2: fix get_parent_stats to have static typing
v3: simplify get_parent_stats to return a StatsEntry pointer instead of
doing it through arguments
v8: replace errors with warnings, we should whatch on them only if we
have corresponding error in detect_pid_reuse else they are fine
v9: change stats to inventory image
Signed-off-by: Pavel Tikhomirov <ptikhomirov@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>