boot VMs with UEFI GSoC

[gqlo]

What does this PR do?

This PR adds support boot VMs with UEFI.

What issues does this PR fix or reference?

Fixes: a bug on removing existing boot parameters

Previous Behavior

Removing an existing boot parameters by passing an None object would fail. ElementTree.remove(parent_tag, tag) because ElementTree does not have a remove attribute.
Apart from that, we also need to make sure None object will not be passed to check_remote(orig_boot.get(key))

New Behavior

User is expected to specify loader and nvram with the corresponding firmware path to boot the VM with UEFI. For instance:

- boot:
     loader: /usr/share/OVMF/OVMF_CODE.fd
     nvram: /usr/share/OVMF/OVMF_VARS.ms.fd

To remove existing boot parameters, pass None object instead.

- boot:
     kernel: None
     initrd: None

Docs & changelog written?

Yes

• Docs
• Changelog

Tests written?

[NOTICE] Bug fixes or features added to Salt require tests.
Please review the test documentation for details on how to implement tests into Salt's test suite.

Yes

Commits signed with GPG?

Yes

Please review Salt's Contributing Guide for best practices.

See GitHub's page on GPG signing for more information about signing commits with GPG.

[dwoz]

@MeAndTheFirefly Nice work, thanks for the contribution.

[gqlo]

@MeAndTheFirefly Nice work, thanks for the contribution.
Thanks. :)