Copy libvirt log file after tempest run

Gate tests fail sometimes due to libvirt problems, but it is difficult
to investigate their reasons or workarounds because there is not any
log about libvirt.
This patch adds libvirt log file to the gate test directory for digging
libvirt problems in the future.

Change-Id: I8771f825fd3ec9df1ba3053c45ce32296aff0fc4
Related-Bug: #1254872
Merge "Copy libvirt log file after tempest run"