schema: Update domain XML schema

'boot' tag shouldn't be exclusive with 'kernel', 'initrd', and 'cmdline',
though the boot sequence doesn't make sense when the guest boots from
kernel directly. But it's useful if booting from kernel is to install
a newguest, even if it's not to install a guest, there is no hurt. And
on the other hand, we allow 'boot' and the kernel tags when parsing.
(cherry picked from commit b34324fb)

docs/schemas/domaincommon.rng

@@ -222,12 +222,14 @@
             <ref name="absFilePath"/>
           </element>
         </optional>
-        <choice>
+        <optional>
           <ref name="osbootkernel"/>
+        </optional>
+        <optional>
           <zeroOrMore>
             <ref name="osbootdev"/>
           </zeroOrMore>
-        </choice>
+        </optional>
         <optional>
           <element name="bootmenu">
             <attribute name="enable">