Add efi_format options to efi_config

[giosh94mhz]

Using this option is possible to create qcow2 EFI disks, instead of the default raw backend.

In previous version of the plugin (with packer 1.9 IIRC), there was a "global" option format = "qcow2" which applied also to EFi disks. When the configuration was changed to disks and efi sections, we lost the ability to set disk to qcow format.

Having EFI disk in qcow2 format is important, because the raw disks prevent snapshot to be taken.

I've searched the tests suite for EFI configuration to attach my changes, but I found none. I'm sorry, but I'm not quite sure how to write a new test, so I had to leave that to you. Of course, I tested the plug-in on our infrastructure (Proxmox 7.4 + Packer 1.11)