virtiofsd: add -o source=PATH to help output

The -o source=PATH option will be used by most command-line invocations.
Let's document it!

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>

tools/virtiofsd/passthrough_ll.c

@@ -1319,6 +1319,7 @@ int main(int argc, char *argv[])
     if (opts.show_help) {
         printf("usage: %s [options]\n\n", argv[0]);
         fuse_cmdline_help();
+        printf("    -o source=PATH             shared directory tree\n");
         fuse_lowlevel_help();
         ret = 0;
         goto err_out1;