Allow one-or-more <boot dev="..."/> entries

According to docs/formatdomain.html.in, "The boot element can be
repeated multiple times to setup a priority list of boot devices to try
in turn." The Relax-NG schema required / allowed exactly one entry.
Signed-off-by: NPhilipp Hahn <hahn@univention.de>

docs/schemas/domain.rng

@@ -115,7 +115,9 @@
         </optional>
         <choice>
           <ref name="osbootkernel"/>
+          <oneOrMore>
             <ref name="osbootdev"/>
+          </oneOrMore>
         </choice>
       </interleave>
     </element>