qemu: support use of virtlogd with file based chardevs
    
    Currently the file based character devices let QEMU write
    directly to a file on disk. This allows a malicious QEMU
    to inflict a denial of service by consuming all free space.
    
    Switch QEMU to use a pipe to virtlogd, which will enforce
    file rollover.
    
    Signed-off-by: Daniel P. Berrange <berrange@redhat.com>