kvm arm64 auto manage nvram

[reedobrien]

This let's libvirt manage qemu's UEFI VARS NVRAM drive. All our docs say we need to create it our selves, but the version of libvirt and qemu that xenial ships with will auto create an nvram disk and remove it when undefined if the --nvram flag is provided.

QA: Get an ARM machine...

1. Run the test suite and ensure then pass
2. juju bootstrap arm64-maas kvm/armtest --build-agent
3. juju add-machine
4. juju add-machine
5. juju deploy ubuntu --to kvm:0
6. juju deploy postgresql --to kvm:0 --constraints "cores=12 mem=4G"
7. juju add-unit postgresql --to kvm:1
   -- ensure everything starts up
8. juju remove application ubuntu
9. juju remove application postgresql
   -- ensure no nvram files were left behind and all the vms were removed without error.
10. juju destroy-controller -y --destroy-all-models kvm/armtest

[reedobrien]

$$merge$$

[jujubot]

Status: merge request accepted. Url: http://juju-ci.vapour.ws:8080/job/github-merge-juju