docs: Create defs.rst.inc as a place to define substitutions

Rather than accumulating generally useful rST substitution
definitions in individual rST files, create a defs.rst.inc
where we can define these. To start with it has the
|qemu_system| definition from qemu-block-drivers.rst.

Since we're making |qemu_system| generally available, add
the ``...`` literal markup so it can be used in running
text as well as inside a parsed-literal block.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

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