Just add the head line to let the editor know it's XML document.
+<?xml version="1.0"?>
<!-- A Relax NG schema for the libvirt capabilities XML format -->
<grammar xmlns="http://relaxng.org/ns/structure/1.0"
datatypeLibrary="http://www.w3.org/2001/XMLSchema-datatypes">
+<?xml version="1.0"?>
<!-- A Relax NG schema for the libvirt node device XML format -->
<grammar xmlns="http://relaxng.org/ns/structure/1.0"
datatypeLibrary="http://www.w3.org/2001/XMLSchema-datatypes">
+<?xml version="1.0"?>
<!-- A Relax NG schema for the libvirt volume encryption XML format -->
<grammar xmlns="http://relaxng.org/ns/structure/1.0"
datatypeLibrary="http://www.w3.org/2001/XMLSchema-datatypes">