qemu: add support for sending QEMU stdout/stderr to virtlogd
    
    Currently the QEMU stdout/stderr streams are written directly to
    a regular file (eg /var/log/libvirt/qemu/$GUEST.log). While those
    can be rotated by logrotate (using copytruncate option) this is
    not very efficient. It also leaves open a window of opportunity
    for a compromised/broken QEMU to DOS the host filesystem by
    writing lots of text to stdout/stderr.
    
    This makes it possible to connect the stdout/stderr file handles
    to a pipe that is provided by virtlogd. The virtlogd daemon will
    read from this pipe and write data to the log file, performing
    file rotation whenever a pre-determined size limit is reached.
    
    Signed-off-by: Daniel P. Berrange <berrange@redhat.com>