Issues in the `storageperf` suite with `AMD64` images

There's a few failures across the suite for Ubuntu here, so I'll break them down by type for ease.

### Testing IOPS

We consistently see failures in `TestSequentialReadIOPS`, `TestSequentialWriteIOPS`, `TestRandomReadIOPS`, across the combinations of machine-type [`c3d-standard-180`, `c4-standard-192`] and disk-type [`pd-balanced`, `hyperdisk-balanced`, `hyperdisk-extreme`].
The errors are that the `...iops average was too low for vm <$combination>: expected at least 0.850000 of target <$target>, got <$result>`.

I've noticed that for these specific machine-type vs. disk-type combinations it consistently fails, but the percentage of expected vs. actual doesn't tend to shift very much (implying to me at least that the method of testing is sound, but the expected values may be set too high).

The `randReadIOPS` , `randWriteIOPS`, `seqReadBW` and `seqWriteBW` values are set in `storageperf/storage_perf_utils.go` and haven't changed from when the repo was first migrated from `guest-test-infra` back in May last year. In the `README.md` it doesn't say that where the values were sourced from, but that "...in a future change, this will be compared to a documented IOPS value".

This could very well be an oversight and the values do indeed need updating to be "less strict", but I'm also curious as to whether you are seeing the same or similar in the debian images? Because of course if the debian images are passing the tests across all the disk-type/machine-type combinations we clearly have some work to do on the Ubuntu side 😄

### `getCPUNvmeMapping` returning an error

This one I'm struggling to narrow down exactly where the issue is, so I'll need to lean on your expertise. From the logs:
```
TestRandomReadIOPS" fails with: "storage_perf_utils.go:544: failed to get hyperdisk additional options:
error could not get cpu to nvme queue mapping: err exit status 1
```
but I can't replicate where the `err` is thrown via `getCPUNvmeMapping` (executing `exec.Command("cat", "/sys/class/block/"+symlinkRealPath+"/mq/*/cpu_list"`) in `getHyperdiskAdditionalOptions`?