ovs-bugtool: Improve how Open vSwitch log files are saved.
This moves the OVS log files from the "network-status" capability, which
has a very small maximum size, to the "system-logs" capability, which is
much larger. It ensures that the newest OVS log files are prioritized over
old versions of system log files (if we just added the OVS log files after
the existing file_output, then new OVS log files would only be included
*after* all archived system log files were added).