Unable to deploy --to kvm:X on focal hosts

[Achilleas Anagnostopoulos]

Attempts to deploy KVM workloads on a focal host (e.g. one obtained via MAAS and added as a manual machine) currently fail. This is because:

1) On focal, virsh is provided by libvirt-client instead of libvirt-bin.
2) Even with virsh installed, we get the following error when attempting to start the virtual machine:

2020-06-15 16:38:31 DEBUG juju.container.kvm run.go:36 output: error: Failed to start domain juju-275797-0-kvm-3
error: Requested operation is not valid: format of backing image '/var/lib/juju/kvm/guests/focal-amd64-backing-file.qcow' of image '/var/lib/juju/kvm/guests/juju-275797-0-kvm-3.qcow' was not specified in the image metadata (See https://libvirt.org/kbase/backing_chains.html for troubleshooting).

According to https://bugzilla.redhat.com/show_bug.cgi?id=1801219, when we create the image we also now need to specify the backing image format via the '-F' (capital F) flag.

[Achilleas Anagnostopoulos (achilleasa)]

PR https://github.com/juju/juju/pull/11714 includes a drive-by fix for this. We should probably back-port the fix to 2.7/2.8

[Achilleas Anagnostopoulos (achilleasa)]

The following PRs back-port the fix to 2.7 and 2.8:
- https://github.com/juju/juju/pull/11718
- https://github.com/juju/juju/pull/11719