Convert docs/specs/ivshmem-spec.txt to rST format.

In converting, I have dropped the sections on the device's command
line interface and usage, as they are already covered by the
user-facing docs in system/devices/ivshmem.rst.

I have also removed the reference to Memnic, because the URL is dead
and a web search suggests that whatever this was it's pretty much
sunk without trace.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 20230927151205.70930-4-peter.maydell@linaro.org
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>