tests/guests: Add some systemd journal data to the Fedora test images.

The journal data blob comes from a real Fedora 20 (x86-64) machine.
New APIs: Add systemd journal support (RHBZ#988100).

This adds simple support for reading the journal files from guests
that use the systemd journal.

The new APIs are:

  journal-open
  journal-close
  journal-next
  journal-skip
  journal-get
  journal-get-data-threshold
  journal-set-data-threshold
  internal-journal-get

More complex journal support (eg. for seeking within the journal,
support for cursors) could be added later.
tests: Add tests for the new journal APIs (RHBZ#988100).

This relies on the test data from
commit b502197ec4f201fa79255243fa09343cf3550ab5.
Version 1.23.11.