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>