USB: Improve usbdevice error messages

When an non-existent USB device is specified on the command line,
print "qemu: could not add USB device 'X'".
Likewise for the usb_{add,del} monitor commands.
Signed-off-by: NScott Tsai <scottt.tw@gmail.com>
Signed-off-by: NAurelien Jarno <aurelien@aurel32.net>

vl.c

@@ -2689,17 +2689,28 @@ static int usb_device_del(const char *devname)
 
 static int usb_parse(const char *cmdline)
 {
-    return usb_device_add(cmdline, 0);
+    int r;
+    r = usb_device_add(cmdline, 0);
+    if (r < 0) {
+        fprintf(stderr, "qemu: could not add USB device '%s'\n", cmdline);
+    }
+    return r;
 }
 
 void do_usb_add(Monitor *mon, const QDict *qdict)
 {
-    usb_device_add(qdict_get_str(qdict, "devname"), 1);
+    const char *devname = qdict_get_str(qdict, "devname");
+    if (usb_device_add(devname, 1) < 0) {
+        qemu_error("could not add USB device '%s'\n", devname);
+    }
 }
 
 void do_usb_del(Monitor *mon, const QDict *qdict)
 {
-    usb_device_del(qdict_get_str(qdict, "devname"));
+    const char *devname = qdict_get_str(qdict, "devname");
+    if (usb_device_del(devname) < 0) {
+        qemu_error("could not delete USB device '%s'\n", devname);
+    }
 }
 
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