Fix NETTX and NETRX negative values, polling time and accumulate values for vCenter
|Affected Versions:||OpenNebula 5.2|
vCenter's Performance Manager may give negative values if it thinks they're not still valid, these values should be ignored.
NETTX and NETRX should be accumulated with previous monitoring values.
Improve number of samples between polls using LAST_MON to track last time information was gathered from vCenter's performance manager.