Most of docs/barrier.txt is describing the protocol implemented
by the input-barrier device. Move this into the interop
section of the manual, and rstify it.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Message-id: 20210727204112.12579-2-peter.maydell@linaro.org