qemu: systemtap_tracing - Change a variant name to avoid conflicts

Signed-off-by: NLucas Meneghel Rodrigues <lmr@redhat.com>

qemu/tests/cfg/systemtap_tracing.cfg

@@ -89,7 +89,7 @@
                 - scsi_req_parsed:
                     probe_var_key = target lun tag cmd mode xfer
                     stap_script_file = scsi_req_parsed.stp
-        -@usb:
+        -@usb_d:
             usb_devices = tablet1 hub
             usbs = usb1 usb2
             usb_type_usb1 = ich9-usb-uhci1