fix(unikernels): use actual subnet mask in Mirage IP configuration instead of hardcoded /24

[mdryaan]

Description

Mirage.Init() reads the subnet mask from data.Net.Mask but ignores it, always hardcoding /24 into the IP address
string. Any network with a prefix other than /24 produces a wrong kernel command line, causing incorrect routing inside
the MirageOS unikernel.

The fix calls subnetMaskToCIDR() — already present in the same package and used correctly by Hermit.Init() and
Mewz.Init() — and uses the returned CIDR prefix instead of the hardcoded /24.

Related issues

• Fixes use actual subnet mask in Mirage IP configuration instead of hardcoded /24 #693

How was this tested?

Added TestMirageInitSubnetMask in pkg/unikontainers/unikernels/mirage_test.go with three sub-tests: a non-/24 mask (255.255.255.240 → /28), a /24 mask (regression check), and no network (empty mask guard).

Before fix

After fix:

LLM usage

N/A

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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[cmainas]

There is something wrong with the PR description. Please if you use LLMs you must disclose it. This is important for various reasons.

[mdryaan]

There is something wrong with the PR description. Please if you use LLMs you must disclose it. This is important for various reasons.

Hi @cmainas , sorry about the messy description I didn't use any LLM to write the code or tests, but I did use GPT-5 to
help format the PR description following the template and forgot to clean it up properly before submitting. I've read
the LLM policy, If I use LLMs for code in the future, I’ll make sure to disclose it properly

[cmainas]

Thank you @mdryaan .