Generate progress messages during launch.

Generate progress messages during launch.

This commit generates approximate progress messages during the
guestfs_launch call.  Currently this code generates:

   0 / 12: launch clock starts
   3 / 12: appliance created
   6 / 12: detected that guest kernel started
   9 / 12: detected that /init script is running
  12 / 12: launch completed successfully

(Note this is not an ABI and may be changed or removed in a future
version).

Progress messages are only generated at all if 5 seconds have elapsed
since the launch, and they are only generated for the ordinary
appliance (not if using attach-method to attach to an existing virtio
serial port).