libxl: increase usbdevice list only when finding such an input device

If passing an empty usbdevice_list to libxl, qemu will always get an
-usb parameter for HVM guests with only non-USB input devices. This
causes qemu to crash when passing pvusb device on HVM guests.

The solution is to allocate the list only when an item to put in it
is found.

src/libxl/libxl_conf.c

@@ -298,6 +298,7 @@ libxlMakeDomBuildInfo(virDomainDefPtr def,
     libxl_domain_build_info *b_info = &d_config->b_info;
     int hvm = def->os.type == VIR_DOMAIN_OSTYPE_HVM;
     size_t i;
+    size_t nusbdevice = 0;
 
     libxl_domain_build_info_init(b_info);
 
@@ -469,47 +470,55 @@ libxlMakeDomBuildInfo(virDomainDefPtr def,
         libxl_defbool_set(&b_info->u.hvm.vnc.enable, 0);
         libxl_defbool_set(&b_info->u.hvm.sdl.enable, 0);
 
-        if (def->ninputs) {
+        for (i = 0; i < def->ninputs; i++) {
+            char **usbdevice;
+
+            if (def->inputs[i]->bus != VIR_DOMAIN_INPUT_BUS_USB)
+                continue;
+
 #ifdef LIBXL_HAVE_BUILDINFO_USBDEVICE_LIST
-            if (VIR_ALLOC_N(b_info->u.hvm.usbdevice_list, def->ninputs+1) < 0)
+            if (VIR_EXPAND_N(b_info->u.hvm.usbdevice_list, nusbdevice, 1) < 0)
                 return -1;
 #else
-            if (def->ninputs > 1) {
+            if (i > 1) {
                 virReportError(VIR_ERR_CONFIG_UNSUPPORTED, "%s",
                         _("libxenlight supports only one input device"));
                 return -1;
             }
 #endif
-            for (i = 0; i < def->ninputs; i++) {
-                char **usbdevice;
-
-                if (def->inputs[i]->bus != VIR_DOMAIN_INPUT_BUS_USB)
-                    continue;
 
 #ifdef LIBXL_HAVE_BUILDINFO_USBDEVICE_LIST
-                usbdevice = &b_info->u.hvm.usbdevice_list[i];
+            usbdevice = &b_info->u.hvm.usbdevice_list[i];
 #else
-                usbdevice = &b_info->u.hvm.usbdevice;
+            usbdevice = &b_info->u.hvm.usbdevice;
 #endif
-                switch (def->inputs[i]->type) {
-                    case VIR_DOMAIN_INPUT_TYPE_MOUSE:
-                        VIR_FREE(*usbdevice);
-                        if (VIR_STRDUP(*usbdevice, "mouse") < 0)
-                            return -1;
-                        break;
-                    case VIR_DOMAIN_INPUT_TYPE_TABLET:
-                        VIR_FREE(*usbdevice);
-                        if (VIR_STRDUP(*usbdevice, "tablet") < 0)
-                            return -1;
-                        break;
-                    default:
-                        virReportError(VIR_ERR_CONFIG_UNSUPPORTED, "%s",
-                                _("Unknown input device type"));
+            switch (def->inputs[i]->type) {
+                case VIR_DOMAIN_INPUT_TYPE_MOUSE:
+                    VIR_FREE(*usbdevice);
+                    if (VIR_STRDUP(*usbdevice, "mouse") < 0)
                         return -1;
-                }
+                    break;
+                case VIR_DOMAIN_INPUT_TYPE_TABLET:
+                    VIR_FREE(*usbdevice);
+                    if (VIR_STRDUP(*usbdevice, "tablet") < 0)
+                        return -1;
+                    break;
+                default:
+                    virReportError(VIR_ERR_CONFIG_UNSUPPORTED, "%s",
+                            _("Unknown input device type"));
+                    return -1;
             }
         }
 
+#ifdef LIBXL_HAVE_BUILDINFO_USBDEVICE_LIST
+        /* NULL-terminate usbdevice_list */
+        if (nusbdevice > 0 &&
+            VIR_EXPAND_N(b_info->u.hvm.usbdevice_list, nusbdevice, 1) < 0) {
+            VIR_DISPOSE_N(b_info->u.hvm.usbdevice_list, nusbdevice);
+            return -1;
+        }
+#endif
+
         /* Allow libxl to calculate shadow memory requirements */
         b_info->shadow_memkb =
             libxl_get_required_shadow_memory(b_info->max_memkb,