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>

tests/genericxml2xmlindata/generic-graphics-vnc-autoport-no.xml

@@ -0,0 +1,30 @@
+<domain type='qemu'>
+  <name>QEMUGuest1</name>
+  <uuid>c7a5fdbd-edaf-9455-926a-d65c16db1809</uuid>
+  <memory unit='KiB'>219100</memory>
+  <currentMemory unit='KiB'>219100</currentMemory>
+  <vcpu placement='static'>1</vcpu>
+  <os>
+    <type arch='i686' machine='pc'>hvm</type>
+    <boot dev='hd'/>
+  </os>
+  <clock offset='utc'/>
+  <on_poweroff>destroy</on_poweroff>
+  <on_reboot>restart</on_reboot>
+  <on_crash>destroy</on_crash>
+  <devices>
+    <emulator>/usr/bin/qemu</emulator>
+    <controller type='usb' index='0'/>
+    <controller type='ide' index='0'/>
+    <controller type='pci' index='0' model='pci-root'/>
+    <input type='mouse' bus='ps2'/>
+    <input type='keyboard' bus='ps2'/>
+    <graphics type='vnc' autoport='no'>
+      <listen type='address'/>
+    </graphics>
+    <video>
+      <model type='cirrus' vram='16384' heads='1' primary='yes'/>
+    </video>
+    <memballoon model='none'/>
+  </devices>
+</domain>