Libvirt - collect system logs as a step LogFile

When an error occurs, displaying system logs in standard output may not be beneficial if they do not provide additional information that could help with debugging the issue.

This was initially implemented as a trap, triggered whenever the scripts exit with a failure. See: MDBF-1130 (https://github.com/MariaDB/buildbot/commit/bbb5955f9f333ffc45d0c836b83dc26357ce7d67)

Sometimes, the errors of interest are located at the beginning of the log, but they are obscured by a large number of system logs. Collecting system logs could be done into a file that is uploaded at the buildbot step level.

This can be implemented by defining the logfiles parameter at the step level. See: https://docs.buildbot.net/latest/manual/configuration/steps/shell_command.html