Do not log rotate very small logs

Without this, after few weeks without use, each defined domain grows a
tail of empty gzipped logs, instead of keeping just the last log of
interest.

* daemon/libvirtd.logrotate.in: only rotate when the log is over 100 KBytes

daemon/libvirtd.logrotate.in

@@ -5,4 +5,5 @@
         compress
         delaycompress
         copytruncate
+        minsize 100k
 }