conf: Parse and format nested-hv feature

Signed-off-by: Andrea Bolognani <abologna@redhat.com>

tests/qemuxml2argvdata/pseries-features.xml

@@ -11,6 +11,7 @@
       <maxpagesize unit='GiB'>1</maxpagesize>
     </hpt>
     <htm state='on'/>
+    <nested-hv state='off'/>
   </features>
   <devices>
     <emulator>/usr/bin/qemu-system-ppc64</emulator>

tests/qemuxml2xmloutdata/pseries-features.xml

@@ -13,6 +13,7 @@
       <maxpagesize unit='KiB'>1048576</maxpagesize>
     </hpt>
     <htm state='on'/>
+    <nested-hv state='off'/>
   </features>
   <clock offset='utc'/>
   <on_poweroff>destroy</on_poweroff>