unikernels/rumprun: add support for multiple block devices

[Rucha0901]

Description

This PR adds support for multiple block devices in Rumprun unikernels running over Solo5 hypervisors (solo5-hvt and solo5-spt).

Key changes:

1. Updated Rumprun struct to retain all block devices passed in UnikernelParams.Block.
2. Updated MonitorBlockCli in pkg/unikontainers/unikernels/rumprun.go to iterate over all devices in UnikernelParams.Block.
3. Auto-assigns ID rootfs to the 0-th block device (if non-explicit) and vol<index> to secondary block devices, while preserving any custom ID set in BlockDevParams.
4. Added unit tests in pkg/unikontainers/unikernels/rumprun_test.go covering single block, multi-block, custom IDs, and empty block configurations.

Related issues

• Fixes [Feature]: Support Multiple Block Devices in Rumprun / Solo5 #887

How was this tested?

• Added new unit test suite TestRumprunMonitorBlockCli in pkg/unikontainers/unikernels/rumprun_test.go.
• Verified Linux cross-compilation of unit test packages:
  GOOS=linux go test -c ./pkg/unikontainers/unikernels

LLM usage

• Antigravity AI assistant

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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