import: rename "gpt" disk image type to "raw"

After all, nspawn can now dissect MBR partition levels, too, hence
".gpt" appears a misnomer. Moreover, the the .raw suffix for these files
is already pretty popular (the Fedora disk images use it for example),
hence sounds like an OK scheme to adopt.

src/nspawn/nspawn.c

@@ -3321,7 +3321,7 @@ static int determine_names(void) {
                                 return -ENOENT;
                         }
 
-                        if (i->type == IMAGE_GPT)
+                        if (i->type == IMAGE_RAW)
                                 r = set_sanitized_path(&arg_image, i->path);
                         else
                                 r = set_sanitized_path(&arg_directory, i->path);