VMware vSphere: Bug fixes

- Fixed bugs in meter conversions i.e. converting the stat value queried
from vSphere to the one used for creating ceilometer samples.

- Switched the vSphere networking counters 'bytesTx/bytesRx' to
'transmitted/received'. The counters are synonymous, but the latter ones
are available since ESX 3.5 and former are available only in ESX 5.0
onwards.

- Added a null check in VsphereOperations._init_vm_moid_lookup_map()

Change-Id: I8691ce4ded7bdc824bd56427642f9dd87081727a
Implements: blueprint vmware-vcenter-server
Merge "VMware vSphere: Bug fixes"