Get previous disk usage only from existing machine configs

[abhay-krishna]

During upgrade of an EKS-A vSphere cluster, one of the preflight checks we do is to validate whether the cumulative disk size requirements of the VSphereMachineConfigs can be met by the vCenter datastore. For this, we get the previous disk size requirement for each VSphereMachineConfig and then add it to the value in the new cluster config, and we sum this value across all VSphereMachineConfigs and compare it to the available space in the datastore. The previous disk usage is obtained by performing a kubectl get call on the VSphereMachineConfig object in the old cluster, but we are also doing this for newly added VSphereMachineConfig which don't exist on the old cluster, which leads to errors like

Error: "Validation failed {"validation": "vsphere provider validation", "error": "validating vsphere machine configs datastore usage: calculating datastore usage: getting eksa vsphere cluster Error from server (NotFound): [vspheremachineconfigs.anywhere.eks.amazonaws.com](http://vspheremachineconfigs.anywhere.eks.amazonaws.com/) \"new-vsphere-machine-config\" not found\n", "remediation": ""}"

We should try to fetch the object only if the previous count of the corresponding WorkerNodeConfiguration is greater than 0, which indicates that the VSphereMachineConfig exists on the old cluster.

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[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (8382d1a) 75.59% compared to head (7e4d86a) 75.60%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6699   +/-   ##
=======================================
  Coverage   75.59%   75.60%           
=======================================
  Files         474      474           
  Lines       38250    38251    +1     
=======================================
+ Hits        28916    28920    +4     
+ Misses       7726     7724    -2     
+ Partials     1608     1607    -1     

Files Changed	Coverage Δ
pkg/providers/vsphere/vsphere.go	68.82% <100.00%> (+0.39%)	⬆️

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[abhay-krishna]

/approve
/backport release-0.17

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[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: abhay-krishna

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[abhay-krishna]

/cherrypick release-0.17

[eks-distro-pr-bot]

@abhay-krishna: new pull request created: #6700

In response to this:

/cherrypick release-0.17

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