inspect: Fix bogus warning for partitions without /boot.ini

Fix a bogus warning introduced by
5abb196de869cd27a6fa2377d79b9a267120f48e. If a non-windows partition
hasn't been detected as something else and falls through to windows
detection, inspection would display a bogus warning about missing
/boot.ini.

RWMJ: Test if guestfs_is_file > 0, in case it returns an error.