support for custom domain xml and advanced KVM cases

For advanced cases where user might want deeper control over KVM domain XML definition, it would be nice to allow the user to supply a known, working domain XML definition with exception of some basic things like the image file path. Example XML taken from testing with virt-manager

[windows-good.xml.txt](https://github.com/user-attachments/files/17998725/windows-good.xml.txt)

```
job "WindowsWorkload" {

datacenters = ["edge"]

group "OfficeEdge" {
count = 1
network {
mode = "host"
port "ssh" {
to = 22
}
}
task "WindowsPrintServer" {
driver = "nomad-driver-virt"

artifact {
source = "http://192.168.0.5/windows-good.xml"
}
config {
domain_xml = "local/windows-good.xml"
image = "/var/lib/virt/Win2022_20324.qcow2"
}
resources {
cores = 2
memory = 4000
}
}
}
}
```

Start by tracing how the driver handles the config fields `domain_xml` and `image`, then compare that behavior with the attached `windows-good.xml.txt` example. Done means a job can supply a known domain XML definition while the driver still substitutes the configured image file path for the KVM workload.

Written by the indexing model from the issue text.