fix: use annotation for MirageOS net device name

[pocopepe]

Description

mirageos unikernels declare their network device names at build time through the solo5 manifest. the problem was urunc always hardcoded "service" as the device name, so if you built a unikernel with a different name it would just fail.
i added a new annotation com.urunc.unikernel.netDev so image builders can set the name at build time and urunc picks it up at runtime. if the annotation isn't there it falls back to "service" so existing images still work.

block device naming and multi network support i left for follow up prs as discussed.

Related issues

• Fixes Support multiple block and network devices over Solo5/MirageOS #315

How was this tested?

go test ./pkg/unikontainers/unikernels/... -v -run TestMirageNetDevName

=== RUN   TestMirageNetDevName
=== RUN   TestMirageNetDevName/uses_net_device_name_from_annotation
=== PAUSE TestMirageNetDevName/uses_net_device_name_from_annotation
=== RUN   TestMirageNetDevName/falls_back_to_service_when_annotation_is_absent
=== PAUSE TestMirageNetDevName/falls_back_to_service_when_annotation_is_absent
=== CONT  TestMirageNetDevName/uses_net_device_name_from_annotation
=== CONT  TestMirageNetDevName/falls_back_to_service_when_annotation_is_absent
--- PASS: TestMirageNetDevName (0.00s)
    --- PASS: TestMirageNetDevName/uses_net_device_name_from_annotation (0.00s)
    --- PASS: TestMirageNetDevName/falls_back_to_service_when_annotation_is_absent (0.00s)
PASS
ok      github.com/urunc-dev/urunc/pkg/unikontainers/unikernels (cached)

LLM usage

Claude Sonnet 4.6

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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