qemu: logrotate: drop minsize directive
    
    On a cloud host it is possible to create 100's of unique instances
    per day, each leaving behind a /var/log/libvirt/qemu/instance-name.log
    file that is < 100k. With the current 'minsize 100k' directive, these
    files are never rotated and hence never removed. Over months of time,
    tens of thousands of these files can accumulate on the host.
    
    Dropping 'minsize 100k' allows rotating small files, which will
    increase the number of log files, but 'rotate 4' ensures they will
    be removed after a month.
    
    Signed-off-by: Jim Fehlig <jfehlig@suse.com>
    Reviewed-by: Daniel P. Berrange <berrange@redhat.com>