qemu: include hostname in QEMU log files
    
    Often when debugging bug reports one is given a copy of the file
    from /var/log/libvirt/qemu/$NAME.log along with other supporting
    files. In a number of cases I've been given sets of files which
    were from different machines. Including the hostname in the QEMU
    log file will help identify when the bug reporter is providing
    bad information.
    
    Signed-off-by: Daniel P. Berrange <berrange@redhat.com>