qemu: Add xhci support

qemu 1.1 features a xhci controller,
this patch adds support for it.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>

tests/qemuhelptest.c

@@ -753,7 +753,8 @@ mymain(void)
             QEMU_CAPS_SCSI_CD,
             QEMU_CAPS_IDE_CD,
             QEMU_CAPS_NO_USER_CONFIG,
-            QEMU_CAPS_HDA_MICRO);
+            QEMU_CAPS_HDA_MICRO,
+            QEMU_CAPS_NEC_USB_XHCI);
 
     return ret == 0 ? EXIT_SUCCESS : EXIT_FAILURE;
 }