conf: Remove duplicate entries in <metadata> by namespace

Since the APIs support just one element per namespace and while
modifying an element all duplicates would be removed, let's do this
right away in the post parse callback.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1190590

tests/qemuxml2xmloutdata/qemuxml2xmlout-metadata-duplicate.xml

@@ -0,0 +1,31 @@
+<domain type='qemu'>
+  <name>QEMUGuest1</name>
+  <uuid>c7a5fdbd-edaf-9455-926a-d65c16db1809</uuid>
+  <metadata>
+    <app1:foo xmlns:app1="http://foo.org/">fooish</app1:foo>
+    <app2:bar xmlns:app2="http://bar.com/" maman="baz">barish</app2:bar>
+  </metadata>
+  <memory unit='KiB'>219100</memory>
+  <currentMemory unit='KiB'>219100</currentMemory>
+  <vcpu placement='static' cpuset='1-4,8-20,525'>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>
+    <disk type='block' device='disk'>
+      <source dev='/dev/HostVG/QEMUGuest1'/>
+      <target dev='hda' bus='ide'/>
+      <address type='drive' controller='0' bus='0' target='0' unit='0'/>
+    </disk>
+    <controller type='ide' index='0'/>
+    <controller type='usb' index='0'/>
+    <controller type='pci' index='0' model='pci-root'/>
+    <memballoon model='virtio'/>
+  </devices>
+</domain>