Add support for limiting guest coredump

Sometimes when guest machine crashes, coredump can get huge due to the
guest memory. This can be limited using madvise(2) system call and is
being used in QEMU hypervisor. This patch adds an option for configuring
that in the domain XML and related documentation.

docs/formatdomain.html.in

@@ -515,9 +515,15 @@
         However, the value will be rounded up to the nearest kibibyte
         by libvirt, and may be further rounded to the granularity
         supported by the hypervisor.  Some hypervisors also enforce a
-        minimum, such as
-        4000KiB. <span class='since'><code>unit</code> since
-        0.9.11</span></dd>
+        minimum, such as 4000KiB.
+
+        In the case of crash, optional attribute <code>dumpCore</code>
+        can be used to control whether the guest memory should be
+        included in the generated coredump or not (values "on", "off").
+
+        <span class='since'><code>unit</code> since 0.9.11</span>,
+        <span class='since'><code>dumpCore</code> since 0.10.2
+        (QEMU only)</span></dd>
       <dt><code>currentMemory</code></dt>
       <dd>The actual allocation of memory for the guest. This value can
         be less than the maximum allocation, to allow for ballooning

docs/schemas/domaincommon.rng

@@ -470,6 +470,14 @@
     <interleave>
       <element name="memory">
         <ref name='scaledInteger'/>
+        <optional>
+          <attribute name="dumpCore">
+            <choice>
+              <value>on</value>
+              <value>off</value>
+            </choice>
+          </attribute>
+        </optional>
       </element>
       <optional>
         <element name="currentMemory">