qemu: add usb-serial support

Add an optional 'type' attribute to <target> element of serial port
device. There are two choices for its value, 'isa-serial' and
'usb-serial'. For backward compatibility, when attribute 'type' is
missing the 'isa-serial' will be chosen as before.

Libvirt XML sample

    <serial type='pty'>
      <target type='usb-serial' port='0'/>
      <address type='usb' bus='0' port='1'/>
    </serial>

qemu commandline:

qemu ${other_vm_args}              \
    -chardev pty,id=charserial0    \
    -device usb-serial,chardev=charserial0,id=serial0,bus=usb.0,port=1

docs/formatdomain.html.in

@@ -3688,7 +3688,14 @@ qemu-kvm -net nic,model=? /dev/null
     <p>
       <code>target</code> can have a <code>port</code> attribute, which
       specifies the port number. Ports are numbered starting from 0. There are
-      usually 0, 1 or 2 serial ports.
+      usually 0, 1 or 2 serial ports. There is also an optional
+      <code>type</code> attribute <span class="since">since 1.0.2</span>
+      which has two choices for its value, one is< code>isa-serial</code>,
+      the other is <code>usb-serial</code>. If <code>type</code> is missing,
+      <code>isa-serial</code> will be used by default. For <code>usb-serial</code>
+      an optional sub-element <code>&lt;address&gt;</code> with
+      <code>type='usb'</code> can tie the device to a particular controller,
+      <a href="#elementsAddress">documented above</a>.
     </p>
 
     <h6><a name="elementCharConsole">Console</a></h6>

docs/schemas/domaincommon.rng

@@ -2473,12 +2473,26 @@
     </attribute>
   </define>
 
+  <define name='qemucdevSerialTgtType'>
+    <attribute name='type'>
+      <choice>
+        <value>isa-serial</value>
+        <value>usb-serial</value>
+      </choice>
+    </attribute>
+  </define>
+
   <define  name="qemucdevTgtDef">
     <element name="target">
       <interleave>
-        <optional>
-          <ref name="qemucdevConsoleTgtType"/>
-        </optional>
+        <choice>
+          <optional>
+            <ref name="qemucdevConsoleTgtType"/>
+          </optional>
+          <optional>
+            <ref name="qemucdevSerialTgtType"/>
+          </optional>
+        </choice>
         <optional>
           <attribute name="port"/>
         </optional>