domain_conf: vnc: preserve autoport value if no port was specified

domain_conf: vnc: preserve autoport value if no port was specified

The issue is that if this graphics definition is provided:

  <graphics type='vnc' port='0'/>

it's parsed as:

  <graphics type='vnc' autoport='no'>
    <listen type='address'/>
  </graphics>

but if the resulting XML is parsed again the output is:

  <graphics type='vnc' port='-1' autoport='yes'>
    <listen type='address'/>
  </graphics>

and this should not happen.  The XML have to always remain the same
after it was already parsed by libvirt.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1383039

Signed-off-by: Pavel Hrdina <phrdina@redhat.com>