hw/arm/nseries: Replace the bluetooth chardev with a "null" chardev

hw/arm/nseries: Replace the bluetooth chardev with a "null" chardev

We are going to remove the bitrotten bluetooth backend, so we can
not use it in the n800 and n810 machines anymore. Replace the chardev
here with a "null" chardev and stop including the "hw/bt.h" header.

I am not aware of a full working N8x0 environment that can be used to
test these machines, but the Linux kernel from the Meego project that
can be found on http://stskeeps.subnetmask.net/meego-n8x0/ still seems
to boot as far as it did before.

Message-Id: <20191120091014.16883-2-thuth@redhat.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>