hw/block/fdc: Kludge missing floppy drive to fix CVE-2021-20196

hw/block/fdc: Kludge missing floppy drive to fix CVE-2021-20196

Guest might select another drive on the bus by setting the
DRIVE_SEL bit of the DIGITAL OUTPUT REGISTER (DOR).
The current controller model doesn't expect a BlockBackend
to be NULL. A simple way to fix CVE-2021-20196 is to create
an empty BlockBackend when it is missing. All further
accesses will be safely handled, and the controller state
machines keep behaving correctly.

Cc: qemu-stable@nongnu.org
Fixes: CVE-2021-20196
Reported-by: Gaoning Pan (Ant Security Light-Year Lab) <pgn@zju.edu.cn>
Reviewed-by: Darren Kenny <darren.kenny@oracle.com>
Reviewed-by: Hanna Reitz <hreitz@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-id: 20211124161536.631563-3-philmd@redhat.com
BugLink: https://bugs.launchpad.net/qemu/+bug/1912780
Resolves: https://gitlab.com/qemu-project/qemu/-/issues/338
Reviewed-by: Darren Kenny <darren.kenny@oracle.com>
Reviewed-by: Hanna Reitz <hreitz@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: John Snow <jsnow@redhat.com>