conf: domain: Restore XPath context after virSecurityDeviceLabelDefParseXML

The function modifies the context but did not care to restore it back.
If a <seclabel> was used on a disk, the <privateData> would not be
parsed.

Use VIR_XPATH_NODE_AUTORESTORE and add a test case to validate that
everything works.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>

tests/qemustatusxml2xmldata/modern-in.xml

@@ -311,6 +311,9 @@
         <backingStore type='file' index='1'>
           <format type='qcow2'/>
           <source file='/var/lib/libvirt/images/base.qcow2'>
+            <seclabel model='dac' relabel='yes'>
+              <label>qemu:qemu</label>
+            </seclabel>
             <reservations managed='yes'>
               <source type='unix' path='/somepath/ux.sck' mode='client'/>
             </reservations>