fix(cloud-hypervisor): pass block devices in a single --disk flag

[CapThunder19]

Description

CloudHypervisor.BuildExecCmd appended a separate --disk flag per block
device. Cloud Hypervisor only accepts one --disk flag with every disk as a
value after it and rejects a repeated flag:

error: the argument '--disk <disk>...' cannot be used multiple times

So any container with more than one block device (e.g. block rootfs + a
block-backed volume) failed to start. A single disk worked, which is why the
existing e2e cases (all single-disk) missed it.

Fix: collect the disk specs and emit them behind one --disk.

# before
--disk path=/rootfs.img,id=rootfs --disk path=/dev/loop0,id=vol0

# after
--disk path=/rootfs.img,id=rootfs path=/dev/loop0,id=vol0

ExactArgs and the no-block-device case are unaffected.

Related issues

• Fixes cloud-hypervisor: container with more than one block device fails to start (repeated --disk flags) #988

How was this tested?

Reproduced end-to-end with urunc + cloud-hypervisor v50.0.0
using a hand-built OCI bundle with a block rootfs and one
loop-backed volume no containerd/devmapper needed. Before the fix,
cloud-hypervisor rejects the args; after, it proceeds to kernel loading.

Also ran go build ./..., go vet, gofmt, and the existing
./pkg/unikontainers/... unit tests all pass

LLM usage

Claude sonnet investigating the bug and fixes

Checklist

• I have read the contribution guide.
• The linter passes locally (make lint) no new issues from this change.
• The e2e tests of at least one tool pass locally (make test_ctr, make test_nerdctl, make test_docker, make test_crictl).
• If LLMs were used: I have read the llm policy.

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[CapThunder19]

pls review it and tell me if any change needed

[Anand-240]

I was going through this PR and the change looks correct to me. On main the block loop adds a --disk for every device, so once there's more than one disk you get multiple --disk flags and cloud-hypervisor just refuses to start. Collecting them under a single --disk is the right fix.
While I was in that folder I noticed cloud-hypervisor doesn't have a test file yet. qemu and hyperlight both have one, so I followed that same pattern.
So I tried writing a small regression test for BuildExecCmd that checks multiple block devices end up under exactly one --disk flag. It fails on main (two flags) and passes with your change, and gofmt and go vet are both clean.
If it's useful, I can send it over. Do you want me to open a PR against your fix/ch-single-disk-flag branch so the fix and the test land together, or should I add it as a follow-up once this merges? Either works for me.

[CapThunder19]

@cmainas why invalid, what changes need to make?

[cmainas]

Hello @CapThunder19 ,

please read the contribution guide.

Specifically:

• the PR template has been changed and this is not allowed.
• No LLM disclosure

[CapThunder19]

@cmainas i have updated the pr description pls check