Fix hyperdisk attach limits

[jsafrane]

What type of PR is this?
/kind bug

What this PR does / why we need it:
Reserve 1 attachment for the root / OS disk when reporting nr. of attachable hyperdisks.

Which issue(s) this PR fixes:
Fixes #1819

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

Fixed reporting of attachment limits of hyperdisks + gen4 VMs - reserving 1 attachment for the OS disk.

[k8s-ci-robot]

@jsafrane: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
pull-gcp-compute-persistent-disk-csi-driver-e2e-windows-2022	0895747	link	false	/test pull-gcp-compute-persistent-disk-csi-driver-e2e-windows-2022

Full PR test history. Your PR dashboard. Please help us cut down on flakes by linking to an open issue when you hit one in your PR.

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[sunnylovestiramisu]

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: jsafrane, sunnylovestiramisu

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[sunnylovestiramisu]

/cherry-pick release-1.17

[k8s-infra-cherrypick-robot]

@sunnylovestiramisu: new pull request created: #2060

In response to this:

/cherry-pick release-1.17

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