vl.c: Fix broken -usb option

Commit 094b287f accidentally broke the "-usb" command line
option, so it would have no effect if the user had not specified
any machine options at that point. (the return value from
'qemu_opts_find(qemu_find_opts("machine"), 0);' is NULL if there
are no user specified options, so it is only to be used for
looking up an option, not when trying to set one.) Similarly,
would '-usbdevice' no longer cause USB to default to enabled.

Fix this regression by using the same style of code for forcing
the usb=on machine option that we use for other aliases such as
'-enable-kvm'.
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>
Signed-off-by: NAlexander Graf <agraf@suse.de>

vl.c

@@ -3273,16 +3273,12 @@ int main(int argc, char **argv, char **envp)
                 break;
             }
             case QEMU_OPTION_usb:
-                machine_opts = qemu_opts_find(qemu_find_opts("machine"), 0);
-                if (machine_opts) {
-                    qemu_opt_set_bool(machine_opts, "usb", true);
-                }
+                olist = qemu_find_opts("machine");
+                qemu_opts_parse(olist, "usb=on", 0);
                 break;
             case QEMU_OPTION_usbdevice:
-                machine_opts = qemu_opts_find(qemu_find_opts("machine"), 0);
-                if (machine_opts) {
-                    qemu_opt_set_bool(machine_opts, "usb", true);
-                }
+                olist = qemu_find_opts("machine");
+                qemu_opts_parse(olist, "usb=on", 0);
                 add_device_config(DEV_USB, optarg);
                 break;
             case QEMU_OPTION_device: