virsh: Add VSH_OFLAG_EMPTY_OK for attach-disk command

As the description of removing CDROM media from
  http://wiki.libvirt.org/page/QEMUSwitchToLibvirt#eject_DEV

Add flag 'VSH_OFLAG_EMPTY_OK' to the option 'source' of attach-disk

Then avoid outputting <source> in the XML if 'source' was empty,
rather than trusting libvirt domain_conf.c to understand an empty
string.
Signed-off-by: NXu He Jie <xuhj@linux.vnet.ibm.com>
Signed-off-by: NEric Blake <eblake@redhat.com>

tools/virsh.c

@@ -11569,7 +11569,8 @@ static const vshCmdInfo info_attach_disk[] = {
 
 static const vshCmdOptDef opts_attach_disk[] = {
     {"domain",  VSH_OT_DATA, VSH_OFLAG_REQ, N_("domain name, id or uuid")},
-    {"source",  VSH_OT_DATA, VSH_OFLAG_REQ, N_("source of disk device")},
+    {"source",  VSH_OT_DATA, VSH_OFLAG_REQ | VSH_OFLAG_EMPTY_OK,
+     N_("source of disk device")},
     {"target",  VSH_OT_DATA, VSH_OFLAG_REQ, N_("target of disk device")},
     {"driver",    VSH_OT_STRING, 0, N_("driver of disk device")},
     {"subdriver", VSH_OT_STRING, 0, N_("subdriver of disk device")},
@@ -11754,6 +11755,10 @@ cmdAttachDisk(vshControl *ctl, const vshCmd *cmd)
 
     if (vshCommandOptString(cmd, "source", &source) <= 0)
         goto cleanup;
+    /* Allow empty string as a placeholder that implies no source, for
+     * use in adding a cdrom drive with no disk.  */
+    if (!*source)
+        source = NULL;
 
     if (vshCommandOptString(cmd, "target", &target) <= 0)
         goto cleanup;
@@ -11808,6 +11813,7 @@ cmdAttachDisk(vshControl *ctl, const vshCmd *cmd)
     if (driver || subdriver || cache)
         virBufferAddLit(&buf, "/>\n");
 
+    if (source)
         virBufferAsprintf(&buf, "  <source %s='%s'/>\n",
                           (isFile) ? "file" : "dev",
                           source);