Currently target-i386.rst includes the documentation of the 'pc'
machine model inline. Split it out into its own file, in a
similar way to target-i386.rst; this gives us a place to put
documentation of other i386 machine models, such as 'microvm'.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>