Log Message:
Update to 1.39

Changelog:
- 1.39 released -

Deal with incompatible change in the output format of the "info block"
command in qemu 2.5.0.

Fix bug where the exit status of the shell command specified with the
--run option was not propagated to the exit status of anita.

New public method start_boot, to facilitate writing test cases that
interact with the boot prompt.

The structured log file now also contains informational messages,
of the form info('message').