qcow-efi format with ESP mount and configurable diskSize

[tylerjl]

Hey folks! This PR adds the qcow-efi format based on @ashraffouda's work and @Mic92's feedback in #194. Credit to both of them for the discussing and existing work in the PR, I just consolidated it into my branch since the work in #194 appeared stalled.

Summary of these changes:

• I started wtih the commit from @ashraffouda, which adds the baseline qcow-efi image
• I've been using their work for a while and found a bug that stopped nixos-rebuild ... switch from working due to the missing EFI mount point, so I added /boot, which fixes that.
• @Mic92 mentioned that exposing the disk image size as an option would be a good thing to do. That makes sense, so I wrapped the format in a traditional options = { ... }; config = { ... }; form. The existing format modules do not do any of this so I had to make it up; I didn't choose to nest the diskSize option under any specific namespace/attribute but if there's any sort of preference it's all the same to me.

I'm actively using this format, so I know it works, but if there's anything additional y'all would like to see just let me know.

[RaitoBezarius]

Hey, can I do something to help you get this PR to the finish line? I'm interested into it as I'm running into limitations with the existing qcow biosboot only for x86_64-linux. :)

[tylerjl]

Apologies for letting this languish a bit, I updated to latest master and addressed the feedback that I was able to. Remaining items that I can see:

• Not sure how to best address the question about the ESP partition
• I think upstream master may be slightly differently-structured now, but I can adhere to any changes that need to be brought in. Looks like style(?) may be different and I couldn't find an existing format that accepts options like this PR has for consoles and diskSize but it seems to work?

[Lassulus]

somehow this fell under my radar, but this looks good to me.

[tylerjl]

Thank you so much, @Lassulus!