ephemeral: Add Ignition config injection support

[gursewak1997]

Add support for injecting Ignition configuration files into ephemeral VMs via QEMU's fw_cfg mechanism (x86_64/aarch64) and virtio-blk (s390x/ppc64le). This enables first-boot provisioning for bootc-based systems that use Ignition.

Implementation:

• Architecture-specific config delivery per FCOS docs:
  • x86_64/aarch64: fw_cfg at opt/com.coreos/config
  • s390x/ppc64le: virtio-blk with serial 'ignition', readonly
• Runtime architecture detection for correct method
• Image validation via labels (coreos.ignition or com.coreos.osname)
• Added readonly support for virtio-blk devices
• Test coverage for fw_cfg functionality

The config is successfully passed to the guest (verified via /sys/firmware/qemu_fw_cfg/). Ignition will process it on first boot. For ephemeral testing with pre-built FCOS images, Ignition skips as expected (subsequent boot behavior).

Useful for custom bootc images with Ignition (see bootc initramfs docs) and future to-disk workflows.

Closes: #228

[gemini-code-assist]

Code Review

This pull request adds support for injecting Ignition configuration files into ephemeral VMs. The implementation correctly handles different injection methods for various architectures (fw_cfg for x86_64/aarch64 and virtio-blk for s390x/ppc64le). The changes include updates to QEMU configuration, command-line options, and logic to detect Ignition support in images.

My review includes a couple of suggestions to improve code clarity, reduce duplication, and enhance performance. Specifically, I've recommended refactoring the virtio-blk device addition methods for better maintainability and optimizing the Ignition support check to be more efficient.