conf: allow use of a logfile with chardev backends

conf: allow use of a logfile with chardev backends

Extend the chardev source XML so that there is a new optional
<log/> element, which is applicable to all character device
backend types. For example, to log output of a TCP backed
serial port

    <serial type='tcp'>
      <source mode='connect' host='127.0.0.1' service='9999'/>
      <protocol type='raw'/>
      <log file='/var/log/libvirt/qemu/demo-serial0.log' append='on'/>
      <target port='0'/>
    </serial>

Not all hypervisors will support use of logfiles.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>