Xen base image
This merge request continues upon !6.
It brings the following features:
- Xen base image creation for STP bridges
- Use temporary directories for image mounts
It obsoletes the following requirements:
When creating an STP bridge Pogo will check for the existence of the Xen base image defined in the settings. If it is not found it will create it using
create-xen-base-image.sh. Pogo will use the base image as install source for the STP bridges speeding the installation to one minute per STP bridge (on an English server).
create-xen-base-image.sh basically takes over the old debootrstrap VM installation from Pogo. It also mounts the base image when created to install the systemd unit file for the bridge service and enables it. This way there is no need to systemd-container anymore.
This MR should be merged after !6. Otherwise, it will be hard to review with the other changes cluttering the review.