Correctly release visible_core_count

[slevenick]

Fixes: hashicorp/terraform-provider-google#13097

If this PR is for Terraform, I acknowledge that I have:

• Searched through the issue tracker for an open issue that this either resolves or contributes to, commented on it to claim it, and written "fixes {url}" or "part of {url}" in this PR description. If there were no relevant open issues, I opened one and commented that I would like to work on it (not necessary for very small changes).
• Generated Terraform, and ran make test and make lint to ensure it passes unit and linter tests.
• Ensured that all new fields I added that can be set by a user appear in at least one example (for generated resources) or third_party test (for handwritten resources or update tests).
• Ran relevant acceptance tests (If the acceptance tests do not yet pass or you are unable to run them, please let your reviewer know).
• Read the Release Notes Guide before writing my release note below.

Release Note Template for Downstream PRs (will be copied)

compute: fixed a crash with `google_compute_instance_template` on a newly released field when `advanced_machine_features` was set

[modular-magician]

Hi there, I'm the Modular magician. I've detected the following information about your changes:

Diff report

Your PR generated some diffs in downstreams - here they are.

Terraform GA: Diff ( 2 files changed, 7 insertions(+))
TF Validator: Diff ( 2 files changed, 3 insertions(+), 3 deletions(-))

[rileykarson]

Looks like this fails an existing test (https://ci-oss.hashicorp.engineering/test/3284804160641725973?currentProjectId=GoogleCloud&branch=%3Cdefault%3E) so there's no addtl. test needed to cover.

[modular-magician]

Tests analytics

Total tests: 2280
Passed tests 2033
Skipped tests: 244
Failed tests: 3

Action taken

Triggering VCR tests in RECORDING mode for the tests that failed during VCR. Click here to see the failed tests

TestAccFirebaserulesRelease_BasicRelease|TestAccContainerNodePool_withNodeConfig|TestAccLoggingBucketConfigProject_cmekSettings

[modular-magician]

Tests passed during RECORDING mode:
TestAccFirebaserulesRelease_BasicRelease[Debug log]
TestAccContainerNodePool_withNodeConfig[Debug log]
TestAccLoggingBucketConfigProject_cmekSettings[Debug log]

All tests passed
View the build log or the debug log for each test