Do not set boot device when doing UEFI based QEMU builds

[anish]

Describe the change you are making here!
The boot mode that packer defaults to ( "-boot", "foo") is ignored in UEFI mode. This is a harmless failure on x86_64 but when using aarch64, QEMU will reject this option and bail out with the following error:

2023/01/23 16:55:28 packer-1.8.5 plugin: Started Qemu. Pid: 3594063
2023/01/23 16:55:28 packer-1.8.5 plugin: Qemu stderr: qemu-system-aarch64: no function defined to set boot device list for this architecture

Added a test case for this, also verified working on RHEL9

[lbajolet-hashicorp]

Hi @anish,

Thanks for the PR, the code makes sense to me, I would just like to understand how you inferred that the -boot command was ignored in UEFI builds, I tried building a x86-64 UEFI image locally, and I did not see any warning regarding this being ignored.

From my understanding it would make sense as EFIVars is what handles boot ordering, but I just wanted to have your input here.

I'd be inclined to approve this PR based on the diff, this looks harmless for x86-64, and if it fixes aarch64 builds, it's a good catch.

When you have time though please do share your thoughts, if only for my own curiosity :)

Thanks again for the fix!

[anish]

@lbajolet-hashicorp I tested manually in qemu. whatever boot order you set in efibootmgr takes precedence if you reuse your efivars image. you can also try not passing an efivars image, and it will ignore everything you pass in no matter what you set and just try the default boot order of pxe->disk->cd