The root volume in a systemdisk setup is handled in a special
way. It is not allowed to setup a custom name or mountpoint for
the root volume. Therefore the size of the root volume can be
setup via the @root volume name. This check looks up the volume
setup and searches if there is a configuration for the '/'
mountpoint which would cause the image build to fail
An instance of RuntimeChecker allows to semantically check for
error conditions according to the provided image description
and build options. Related to #20