This provides /dev/pts and other things that are important for
builds (for example, Python 2.x has ‘openpty’ unit tests that can only
succeed when /dev/pts is available.)
* gnu/services/virtualization.scm (%virtual-build-machine-operating-system)
[file-systems]: Add %BASE-FILE-SYSTEMS.
Change-Id: I7d12a4cb491e957bf55e6c5f9dd09c013473ca42