CLOUDSTACK-3851 [VMWare] VM start fails with NPE while trying to retrieve cores per socket

If VMware host version is ESXi 4.1 and vCenter version is 5.0 or later, number of cores per socket of a VM running on ESXi 4.1 host would be undefined as it's not supported for versions prior to 5.0.
Hence expecting undefined/null value for that and handling it.

Signed-off-by: Sateesh Chodapuneedi <sateesh@apache.org>