Commit ace11d3 adds a serial port device to instances, so the serial
console output can be sent to a virtual serial port concentrator (VSPC).
This patch finishes the implementation by returning the output saved by
VSPC to the end user. The config option 'serial_log_dir' should have the
same value as in the VSPC configuration (i.e. it must point to the same
directory).
The VSPC implementation is available at
https://github.com/openstack/vmware-vspc
blueprint vmware-console-log-complete
Change-Id: I7e40dc41b0354d414bc8eae331f8257959e1d123
d9c03b1ecb