qemu: New XML to disable memory merge at guest startup

QEMU introduced command line "-mem-merge=on|off" (defaults to on) to
enable/disable the memory merge (KSM) at guest startup. This exposes
it by new XML:
  <memoryBacking>
    <nosharepages/>
  </memoryBacking>

The XML tag is same with what we used internally for old RHEL.

tests/qemuxml2xmltest.c

@@ -158,6 +158,7 @@ mymain(void)
     DO_TEST("hyperv");
 
     DO_TEST("hugepages");
+    DO_TEST("nosharepages");
     DO_TEST("disk-aio");
     DO_TEST("disk-cdrom");
     DO_TEST("disk-floppy");