Migrate metric queries from libvirt to OpenStack

Example, the **compute overview** dashboard has the following for scheduled VMs.

```
sum by(project_name) (libvirt_domain_info_meta{juju_application=~"$juju_application",juju_model=~"$juju_model",juju_model_uuid=~"$juju_model_uuid",juju_unit=~"$juju_unit",project_name=~"$project"})
```

[Source](https://github.com/canonical/openstack-exporter-operator/blob/cb5ef8fae6d25e87a9bdc7c750e18d3240ad5c43/src/grafana_dashboards/compute.json#L87)

But, the **cloud usage** dashboard has the following for scheduled VMs.

```
sum(openstack_nova_running_vms{hostname=~"$hypervisor.*",juju_application=~"$juju_application",juju_model=~"$juju_model",juju_model_uuid=~"$juju_model_uuid",juju_unit=~"$juju_unit"})
```

[Source](https://github.com/canonical/openstack-exporter-operator/blob/cb5ef8fae6d25e87a9bdc7c750e18d3240ad5c43/src/grafana_dashboards/cloud.json#L144)

The first does not work without the libvirt exporter, and the second does. Making these changes where possible is beneficial due to the [prometheus libvirt exporter](https://charmhub.io/prometheus-libvirt-exporter) being deprecated.