build: Don't fail on '<' or '>' with old xmllint

Older xmllint version don't allow such characters in datatype anyURI.
In order not to change too much, I'm suggesting making a choice of
anyURI or 'absPathName' which should be fine (checked with upstream
and that old xmllint, both work fine).
Signed-off-by: NMartin Kletzander <mkletzan@redhat.com>

docs/schemas/storagevol.rng

@@ -105,7 +105,10 @@
     <element name='target'>
       <optional>
         <element name='path'>
-          <data type='anyURI'/>
+          <choice>
+            <data type='anyURI'/>
+            <ref name='absFilePath'/>
+          </choice>
         </element>
       </optional>
       <ref name='format'/>