Expose extra UEFI BIOS configurations

[onenhansen]

Description Allow defining custom configurations for advanced UEFI settings, such as 'Above 4G Encoding'

Use case Some UEFI settings are useful or necessary for specific use cases, such as PCI Passthrough of a GPU with a high amount of VRAM or multiple GPU's to the same VM.

Interface Changes When selecting UEFI firmware, expose an Advanced Options dropdown to allow configuration of these values. Should just be a Name and Value pair rather than predefined settings.

Additional Context QEMU fw_cfg documentation - https://www.qemu.org/docs/master/specs/fw_cfg.html#externally-provided-items

Example of deployment XML to add this:

    <qemu:commandline>
      <qemu:arg value='-fw_cfg'/>
      <qemu:arg value='opt/ovmf/X-PciMmio64Mb,string=65536'/>
    </qemu:commandline> 

Progress Status

• [ ] Code committed
• [ ] Testing - QA
• [ ] Sunstone
• [ ] Documentation (Release notes - resolved issues, compatibility, known issues)

[rsmontero]

#6989