dissect: optionally, only look for GPT partition tables, nothing else

This is useful for reusing the dissector logic in the gpt-auto-discovery logic:
there we really don't want to use MBR or naked file systems as root device.

src/nspawn/nspawn.c

@@ -3743,7 +3743,7 @@ int main(int argc, char *argv[]) {
                         goto finish;
                 }
 
-                r = dissect_image(loop->fd, arg_root_hash, arg_root_hash_size, &dissected_image);
+                r = dissect_image(loop->fd, arg_root_hash, arg_root_hash_size, 0, &dissected_image);
                 if (r == -ENOPKG) {
                         log_error_errno(r, "Could not find a suitable file system or partition table in image: %s", arg_image);