Introduce <readonly> for hostdev

Since it's generic enough to be used by other types in future, I
put it in <hostdev> as sub-element, though now it's only used by
scsi host device.

docs/formatdomain.html.in

@@ -2415,6 +2415,10 @@
         could be changed in the future with no impact to domains that
         don't specify anything.
       </dd>
+      <dt><code>readonly</code></dt>
+      <dd>Indicates that the device is readonly, only supported by SCSI host
+        device now. <span class="since">Since 1.0.6 (QEMU and KVM only)</span>
+      </dd>
     </dl>
 
 

docs/schemas/domaincommon.rng

@@ -3075,6 +3075,11 @@
         <optional>
           <ref name="address"/>
         </optional>
+        <optional>
+          <element name="readonly">
+            <empty/>
+          </element>
+        </optional>
       </interleave>
     </element>
   </define>