bios: Add support for SGA

This patch creates new <bios> element which, at this time has only the
attribute useserial='yes|no'. This attribute allow users to use
Serial Graphics Adapter and see BIOS messages from the very first moment
domain boots up. Therefore, users can choose boot medium, set PXE, etc.

docs/schemas/domain.rng

@@ -151,6 +151,9 @@
         <optional>
           <ref name="smbios"/>
         </optional>
+        <optional>
+          <ref name="bios"/>
+        </optional>
       </interleave>
     </element>
   </define>
@@ -2277,6 +2280,17 @@
     </element>
   </define>
 
+  <define name="bios">
+    <element name="bios">
+      <attribute name="useserial">
+        <choice>
+          <value>yes</value>
+          <value>no</value>
+        </choice>
+      </attribute>
+    </element>
+  </define>
+
   <define name="address">
     <element name="address">
       <choice>