RFE: Optional `virt-customize` pre-step for appropriate cloud images before booting.

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#### Description

A lot of pre-packaged cloud images in QCOW2 or AMI formats (not ISO) ship without SSH or credentials configured and require customization by the users before booting. It would be nice if we could include an optional pre-boot step for `virt-customize` commands to prepare the images with credentials or passwords before boot. This includes distros like CoreOS and/or Fedora/RHEL Atomic and also some of the Amazon Linux 2 releases.

Examples include setting up users offline before booting an image: http://smoogespace.blogspot.com/2022/02/how-to-install-centos-stream-9-cloud.html

#### Use Case(s)

This is largely helpful with the QEMU builder with `disk_image = false` where you can clone an image and skip the ISO install.

Note this step should happen after disk checksum verification and clone but before boot. It should not write or modify the image cache.

#### Potential configuration

```
virt-customize = [
"--ssh-inject root:file:/home/ssmoogen/.ssh/id_ecdsa.pub",
"--root-password random"
]
```

#### Potential References