Allow balloon driver collection to be adjusted dynamically

Use the virDomainSetMemoryStatsPeriodFlags() to pass a period defined by
usage of a new --period option in order to set the collection period for the
balloon driver. This may enable or disable the collection based on the value.

Add the --current, --live, & --config options to dommemstat.

docs/formatdomain.html.in

@@ -4664,7 +4664,16 @@ qemu-kvm -net nic,model=? /dev/null
         <p>
           The optional <code>period</code> allows the QEMU virtio memory
           balloon driver to provide statistics through the <code>virsh
-          dommemstat [domain]</code> command.
+          dommemstat [domain]</code> command. By default, collection is
+          not enabled. In order to enable, use the <code>virsh dommemstat
+          [domain] --period [number]</code> command or <code>virsh edit</code>
+          command to add the option to the XML definition.
+          The <code>virsh dommemstat</code> will accept the options
+          <code>--live</code>, <code>--current</code>, or <code>--config</code>.
+          If an option is not provided, the change for a running domain will
+          only be made to the active guest.
+          If the QEMU driver is not at the right
+          revision, the attempt to set the period will fail.
           <span class='since'>Since 1.1.1, requires QEMU 1.5</span>
         </p>
       </dd>