[CI] Flaky coverage check failure on unrelated PRs

[rustiqly]

Description

The coverage.Azure.sonic-sairedis.amd64 CI check is failing intermittently on PRs that should not affect coverage.

Affected PR

• [vslib] Fix VS SAI reporting 0xFFFFFFFF oper speed for virtio NICs #1763 (VS oper speed -1 fix)

This PR adds a small fallback path in vs_get_oper_speed() and fixes a failing test. The coverage check should not be impacted by this change.

Impact

Blocks PR merge and wastes contributor time.