xml: Add element <title> to allow short description of domains

This patch adds a new element <title> to the domain XML. This attribute
can hold a short title defined by the user to ease the identification of
domains. The title may not contain newlines and should be reasonably short.

 *docs/formatdomain.html.in
 *docs/schemas/domaincommon.rng
        - add schema grammar for the new element and documentation
  *src/conf/domain_conf.c
  *src/conf/domain_conf.h
        - add field to hold the new attribute
        - add code to parse and create XML with the new attribute

tests/domainschemadata/qemu-simple-description-title.xml

@@ -0,0 +1,27 @@
+<domain type='qemu'>
+  <name>qemu-demo</name>
+  <uuid>603cc28c-9841-864e-0949-8cc7d3bae9f8</uuid>
+  <memory>65536</memory>
+  <currentMemory>65536</currentMemory>
+  <title>A short description of this domain</title>
+  <description>
+      A longer explanation that this domain is a test domain
+      for validating domain schemas.
+  </description>
+  <vcpu>1</vcpu>
+  <os>
+    <type arch='x86_64' machine='pc-0.14'>hvm</type>
+  </os>
+  <features>
+    <acpi/>
+    <apic/>
+    <pae/>
+  </features>
+  <clock offset='utc'/>
+  <on_poweroff>destroy</on_poweroff>
+  <on_reboot>restart</on_reboot>
+  <on_crash>restart</on_crash>
+  <devices>
+    <emulator>/usr/bin/qemu-kvm</emulator>
+  </devices>
+</domain>