The CPU model is a qemu setting that lets users change the default cpu
model used for building their VM image.

On some Linux distributions (RHEL9), the default value for x86-64 hosts
and guests (qemu64) is unsupported, leading to the plugin failing to
start a VM.

To make it easier for these users to user Packer with qemu, we add this
option to the configurations, so they can specify it.

This also allows us to document the potential problem they will
encounter, and suggest a workaround if/when it arises.

Contrary to the original intent, the efivars.fd file that contains the
required keys and EFI entries was removed all the time, including when
the build was successful.

This is an error, and is therefore fixed with this commit.

* example: update EFI build example with build_steps

Since we added the build_steps alternative for documenting and adding
the boot commands, we update the example so it shows how it is used.

Co-authored-by: Wilken Rivera <dev@wilkenrivera.com>