Add persistent guest & hv logging in xenconsoled.

Add persistent guest & hv logging in xenconsoled.

  * The  --log  command line argument takes one of 4 values

      - none   - no logging   (the default)
      - hv     - log all hypervisor messages
      - guest  - log all guest messages
      - both   - log all guest & hypervisor messages

  * The --log-dir command line argument takes a path to specify where
    to store logfiles. If omitted it defaults to /var/log/xen/console

  * The hypervisor logfile is $LOGDIR/hypervisor.log

  * The guest logfile is  $LOGDIR/guest-[NAME].log

  * If receiving a SIGHUP it will close & re-open all log files to
    enable logrotate to do its magic

  * Fixes the permissions of /var/run/xenconsoled.pid

  * Adds a --pid-file command line argument to override the default
    location of pid file (this is not really related to logging, but
    since I was in that code...)

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