qemu: Report better error when memory device source has wrong NUMA node

When starting a VM with hotpluggable memory devices the user may specify
an invalid source NUMA node. Libvirt would pass through the error from
qemu:

 # virsh start test3
 error: Failed to start domain test3
 error: internal error: process exited while connecting to monitor:
 2015-03-25T01:12:17.205913Z qemu-kvm: -object memory-backend-ram,id=memdimm0
 ,size=536870912,host-nodes=1-3,policy=bind: cannot bind memory to host NUMA nodes:
 Invalid argument

This patch adds a check that allows to report better error:

 # virsh start test3
 error: Failed to start domain test3
 error: configuration unsupported: NUMA node 1 is unavailable
Signed-off-by: NLuyao Huang <lhuang@redhat.com>

src/qemu/qemu_command.c

@@ -4757,6 +4757,8 @@ qemuBuildMemoryBackendStr(unsigned long long size,
     }
 
     if (nodemask) {
+        if (!virNumaNodesetIsAvailable(nodemask))
+            goto cleanup;
         if (virJSONValueObjectAdd(props,
                                   "m:host-nodes", nodemask,
                                   "S:policy", qemuNumaPolicyTypeToString(mode),