If you want to connect to the serial port for a libvirt qemu vm, you can edit the VM definition ('virsh edit <vmname>') and add the following:

{{{
    <serial type='tcp'>
      <source mode='bind' host='' service='2222'/>
      <protocol type='raw'/>
      <target port='0'/>
    </serial>
    <console type='tcp'>
      <source mode='bind' host='' service='2222'/>
      <protocol type='raw'/>
      <target type='serial' port='0'/>
    </console>
}}}

Then telnet localhost 2222 to connect to the serial port.