qemu-nbd: regression with arguments passing into nbd_client_thread()

Unfortunately
    commit 03b67621
    Author: Denis V. Lunev <den@openvz.org>
    Date:   Mon Jul 17 16:55:40 2023 +0200
    qemu-nbd: pass structure into nbd_client_thread instead of plain char*
has introduced a regression. struct NbdClientOpts resides on stack inside
'if' block. This specifically means that this stack space could be reused
once the execution will leave that block of the code.

This means that parameters passed into nbd_client_thread could be
overwritten at any moment.

The patch moves the data to the namespace of main() function effectively
preserving it for the whole process lifetime.

Signed-off-by: Denis V. Lunev <den@openvz.org>
CC: Eric Blake <eblake@redhat.com>
CC: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
CC: <qemu-stable@nongnu.org>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-ID: <20230727105828.324314-1-den@openvz.org>
Signed-off-by: Eric Blake <eblake@redhat.com>

qemu-nbd.c

@@ -589,6 +589,9 @@ int main(int argc, char **argv)
     const char *pid_file_name = NULL;
     const char *selinux_label = NULL;
     BlockExportOptions *export_opts;
+#if HAVE_NBD_DEVICE
+    struct NbdClientOpts opts;
+#endif
 
 #ifdef CONFIG_POSIX
     os_setup_early_signal_handling();
@@ -1145,7 +1148,7 @@ int main(int argc, char **argv)
     if (device) {
 #if HAVE_NBD_DEVICE
         int ret;
-        struct NbdClientOpts opts = {
+        opts = (struct NbdClientOpts) {
             .device = device,
             .fork_process = fork_process,
             .verbose = verbose,