cdrom_id: Fix detection of reblanked DVD+RW and DVD-RAM

Once formatted, DVD+RW and DVD-RAM discs are always reported "complete" by the
drive. Check the if the Volume Descriptor or Volume Descriptor Anchor sectors
are empty, and if so, set the status to blank. If the disc is unformatted the
blank status is maintained and no reads are issued. If the disc is formatted and
read command fails, the status remains set to complete to avoid accidental
blanking.

Bug-Ubuntu: https://launchpad.net/bugs/581925

Signed-off-by: Martin Pitt <martin.pitt@ubuntu.com>