conf: introduce 'autodeflate' attribute for memballoon device

Excessive memory balloon inflation can cause invocation of OOM-killer,
when Linux is under severe memory pressure. QEMU memballoon device
has a feature to release some memory at the last moment before some
process will be get killed by OOM-killer.

Introduce a new optional balloon device attribute 'autodeflate' to
enable or disable this feature.

docs/formatdomain.html.in

@@ -5962,6 +5962,16 @@ qemu-kvm -net nic,model=? /dev/null
           <li>'xen' &mdash; default with Xen</li>
         </ul>
       </dd>
+      <dt><code>autodeflate</code></dt>
+      <dd>
+        <p>
+          The optional <code>autodeflate</code> attribute allows to
+          enable/disable (values "on"/"off", respectively) the ability of the
+          QEMU virtio memory balloon to release some memory at the last moment
+          before a guest's process get killed by Out of Memory killer.
+          <span class="since">Since 1.3.1, QEMU and KVM only</span>
+        </p>
+      </dd>
       <dt><code>period</code></dt>
       <dd>
         <p>

docs/schemas/domaincommon.rng

@@ -3420,6 +3420,11 @@
           <value>none</value>
         </choice>
       </attribute>
+      <optional>
+        <attribute name="autodeflate">
+          <ref name="virOnOff"/>
+        </attribute>
+      </optional>
       <interleave>
         <optional>
           <ref name="alias"/>

src/conf/domain_conf.c

@@ -11354,6 +11354,7 @@ virDomainMemballoonDefParseXML(xmlNodePtr node,
                                unsigned int flags)
 {
     char *model;
+    char *deflate;
     virDomainMemballoonDefPtr def;
     xmlNodePtr save = ctxt->node;
     unsigned int period = 0;
@@ -11374,6 +11375,13 @@ virDomainMemballoonDefParseXML(xmlNodePtr node,
         goto error;
     }
 
+    if ((deflate = virXMLPropString(node, "autodeflate")) &&
+        (def->autodeflate = virTristateSwitchTypeFromString(deflate)) <= 0) {
+        virReportError(VIR_ERR_CONFIG_UNSUPPORTED,
+                       _("invalid autodeflate attribute value '%s'"), deflate);
+        goto error;
+    }
+
     ctxt->node = node;
     if (virXPathUInt("string(./stats/@period)", ctxt, &period) < -1) {
         virReportError(VIR_ERR_XML_ERROR, "%s",
@@ -11392,6 +11400,7 @@ virDomainMemballoonDefParseXML(xmlNodePtr node,
 
  cleanup:
     VIR_FREE(model);
+    VIR_FREE(deflate);
 
     ctxt->node = save;
     return def;
@@ -17342,6 +17351,15 @@ virDomainMemballoonDefCheckABIStability(virDomainMemballoonDefPtr src,
         return false;
     }
 
+    if (src->autodeflate != dst->autodeflate) {
+        virReportError(VIR_ERR_CONFIG_UNSUPPORTED,
+                       _("Target balloon autodeflate attribute value "
+                         "'%s' does not match source '%s'"),
+                       virTristateSwitchTypeToString(dst->autodeflate),
+                       virTristateSwitchTypeToString(src->autodeflate));
+        return false;
+    }
+
     if (!virDomainDeviceInfoCheckABIStability(&src->info, &dst->info))
         return false;
 
@@ -20523,6 +20541,11 @@ virDomainMemballoonDefFormat(virBufferPtr buf,
     }
 
     virBufferAsprintf(buf, "<memballoon model='%s'", model);
+
+    if (def->autodeflate != VIR_TRISTATE_SWITCH_ABSENT)
+        virBufferAsprintf(buf, " autodeflate='%s'",
+                          virTristateSwitchTypeToString(def->autodeflate));
+
     virBufferAdjustIndent(&childrenBuf, indent + 2);
 
     if (def->period)

src/conf/domain_conf.h

@@ -1635,6 +1635,7 @@ struct _virDomainMemballoonDef {
     int model;
     virDomainDeviceInfo info;
     int period; /* seconds between collections */
+    int autodeflate; /* enum virTristateSwitch */
 };
 
 struct _virDomainNVRAMDef {