discover-image: don't ignore symlinks to raw images

Since 5c6bb28999 image_discover() uses
chaseat() to chase the path to the image. This however breaks the raw
image check in image_make() as "path" is now not the symlink itself, but
the symlink target.

So with:

$ ls -l /var/lib/machines
total 872104
lrwxrwxrwx. 1 root root         12 Jul 14 06:10 foo.raw -> foo.squashfs
-rw-r--r--. 1 root root 5368709120 Jul 13 02:07 foo.squashfs

The endswith(path, ".raw") check is now performed on "/.../foo.squashfs"
instead of "/.../foo.raw", making it false and thus ignoring the image
symlink completely.

Address this by also checking if the pretty name is set - if so, and the
path is a regular file, the caller must've been image_find() or
image_discover() which already checked if the original path ends in .raw
and is a regular file.

Follow-up for 5c6bb28999.

Resolves: #41656

src/shared/discover-image.c

@@ -618,7 +618,7 @@ static int image_make(
                 (*ret)->foreign_uid_owned = uid_is_foreign(st->st_uid);
                 return 0;
 
-        } else if (S_ISREG(st->st_mode) && endswith(path, ".raw")) {
+        } else if (S_ISREG(st->st_mode) && (endswith(path, ".raw") || pretty)) {
                 usec_t crtime = 0;
 
                 /* It's a RAW disk image */