console: add option to launch telnet session

Over the coming months I would like to add supporting features to webvirtmgr needed for ARM KVM hosts. This is the first piece needed to make webvirtmgr useful with ARM hosts. As of right now, there is no video/graphic devices supported on ARM guests, and so VNC cannot be used for console. The best solution to control the virtual machine is to redirect the serial console over telnet.

    <serial type='tcp'>
      <source mode='bind' host='0.0.0.0' service='4555'/>
      <protocol type='telnet'/>
      <target port='0'/>
      <alias name='serial0'/>
    </serial>
    <console type='tcp'>
      <source mode='bind' host='0.0.0.0' service='4555'/>
      <protocol type='telnet'/>
      <target type='serial' port='0'/>
      <alias name='serial0'/>
    </console>

This change adds the option to launch a telnet console session if one exists.

Here is a quick demo of this feature:

https://docs.google.com/file/d/0B9DbsE2BbZ7uWmNnTHQxYW1IZVE/edit