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>

qemu-deprecated.texi

@@ -128,6 +128,13 @@ The @option{[hub_id name]} parameter tuple of the 'hostfwd_add' and
 The ``ivshmem'' device type is replaced by either the ``ivshmem-plain''
 or ``ivshmem-doorbell`` device types.
 
+@subsection bluetooth (since 3.1)
+
+The bluetooth subsystem is unmaintained since many years and likely bitrotten
+quite a bit. It will be removed without replacement unless some users speaks
+up at the @email{qemu-devel@@nongnu.org} mailing list with information about
+their usecases.
+
 @section System emulator machines
 
 @subsection pc-0.10 and pc-0.11 (since 3.0)

qemu-options.hx

@@ -2772,6 +2772,10 @@ logic.  The Transport Layer is decided by the machine type.  Currently
 the machines @code{n800} and @code{n810} have one HCI and all other
 machines have none.
 
+Note: This option and the whole bluetooth subsystem is considered as deprecated.
+If you still use it, please send a mail to @email{qemu-devel@@nongnu.org} where
+you describe your usecase.
+
 @anchor{bt-hcis}
 The following three types are recognized:
 

vl.c

@@ -3269,6 +3269,10 @@ int main(int argc, char **argv, char **envp)
                 break;
 #endif
             case QEMU_OPTION_bt:
+                warn_report("The bluetooth subsystem is deprecated and will "
+                            "be removed soon. If the bluetooth subsystem is "
+                            "still useful for you, please send a mail to "
+                            "qemu-devel@nongnu.org with your usecase.");
                 add_device_config(DEV_BT, optarg);
                 break;
             case QEMU_OPTION_audio_help: