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/formatdomain.html.in

@@ -86,6 +86,7 @@
     <boot dev='cdrom'/>
     <bootmenu enable='yes'/>
     <smbios mode='sysinfo'/>
+    <bios useserial='yes'/>
   </os>
   ...</pre>
 
@@ -137,6 +138,14 @@
       specified, the hypervisor default is used. <span class="since">
       Since 0.8.7</span>
       </dd>
+      <dt><code>bios</code></dt>
+      <dd>This element has attribute <code>useserial</code> with possible
+        values <code>yes</code> or <code>no</code>. It enables or disables
+        Serial Graphics Adapter which allows users to see BIOS messages
+        on a serial port. Therefore, one needs to have
+        <a href="#elementCharSerial">serial port</a> defined.
+        <span class="since">Since 0.9.4</span>
+      </dd>
     </dl>
 
     <h4><a name="elementsOSBootloader">Host bootloader</a></h4>

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>