feat: Make firmware configurable

[mboutet]

I extracted the changes as-is from #28.

I needed to set the firmware to uefi because bios was not working properly when trying the build an Ubuntu 20.04 image. I had some weird issues when booting because xenbus_probe_frontend was timing out while waiting for devices to initialize. When using uefi, I don't have this problem and the Packer build completes in around 15 minutes instead of 27 minutes.

I'm not well-versed in Go, so let me know if something is missing such as tests or if you need more information from me.

I tested locally using:

go build -o packer-plugin-xenserver

chmod +x packer-plugin-xenserver

mv packer-plugin-xenserver \
  /usr/local/bin/github.com/ddelnano/xenserver/packer-plugin-xenserver_v0.4.0_x5.0_darwin_amd64

sha256sum /usr/local/bin/github.com/ddelnano/xenserver/packer-plugin-xenserver_v0.4.0_x5.0_darwin_amd64 \
  > /usr/local/bin/github.com/ddelnano/xenserver/packer-plugin-xenserver_v0.4.0_x5.0_darwin_amd64_SHA256SUM

Then in the folder with my Packer file:

packer init .
packer build .

[ddelnano]

@mboutet thanks for pulling those changes out. The testing looks good to me, but I'll perform the same verification myself to make sure that I get a working packer build. Once I've verified that we should be good to merge this 👍

[ddelnano]

I was able to get a successful build when specifying uefi so this change looks good to me.

[ddelnano]

v0.4.1 will be released soon including this new functionality.