bt: Mark the bluetooth subsystem as deprecated

bt: Mark the bluetooth subsystem as deprecated

It has been unmaintained since years, and there were only trivial or
tree-wide changes to the related files since many years, so the
code is likely very bitrotten and broken. For example the following
segfaults as soon as as you press a key:

 qemu-system-x86_64 -usb -device usb-bt-dongle -bt hci -bt device:keyboard

Since we are not aware of anybody using bluetooth with the current
version of QEMU, let's mark the subsystem as deprecated, with a special
request for the users to write to the qemu-devel mailing list in case
they still use it (so we could revert the deprecation status in that
case).

Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-id: 1542016830-19189-1-git-send-email-thuth@redhat.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>