test(hypervisors): cover Qemu.BuildExecCmd

[parthdagia05]

Description

Qemu.BuildExecCmd in pkg/unikontainers/hypervisors/qemu.go didn't have any direct unit-test coverage, so this PR adds qemu_test.go that walks through the main branches the function actually exercises:

• Static flags that should always be present
• Memory default fallback and custom values
• vCPU handling (omits -smp when zero, sets it correctly otherwise)
• Seccomp on / off
• Architecture-specific machine type (-M virt on arm64)
• Networking: empty TapDev, generic tap setup, custom MonitorNetCli, and vhost on/off
• Initrd present / absent
• Sharedfs: 9pfs, virtiofs, and none
• Block devices: no blocks, generic, and ExactArgs verbatim path
• MonitorCli extras (ExtraInitrd, OtherArgs)
• vsock with VAccelType set
• The -append rule for the kernel command line

A small fakeUnikernel stub keeps the tests focused on BuildExecCmd itself rather than pulling in any real unikernel implementation.

Related issues

• Fixes #

How was this tested?

• go test ./pkg/unikontainers/hypervisors/... passes locally
• go build ./... passes
• All sub-tests use t.Parallel() and assert against the joined command string

LLM usage

Used Anthropic's Claude Opus 4.6 to help research the function's branches and shape the test cases around the actual code paths.

Checklist

• I have read the contribution guide.
• The linter passes locally (make lint).
• 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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