Add unit tests for QEMU hypervisor backend

[sachin9058]

Description

The QEMU backend under pkg/unikontainers/hypervisors currently has limited unit test coverage.

I would like to improve coverage for QEMU related functionality, mainly around command construction and runtime configuration.

Planned work:

• Add tests for BuildExecCmd()
• Validate command generation for CPU/memory options
• Add networking related scenarios
• Add sharedfs related scenarios
• Improve overall coverage for the QEMU backend

[sachin9058]

I would like to work on this,

[cmainas]

Hello @sachin9058 ,

please do not open new issues for unit tests. We already have an umbrella issue for all of them #96