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.

Add a comment noting a pitfall where putting literal markup in the
definition of |qemu_system| makes it misrender manpage output; this
means the point-of-use must handle the literal markup (which is
almost always done by having it inside a parsed-literal block).

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Kashyap Chamarthy <kchamart@redhat.com>
Tested-by: Alex Bennée <alex.bennee@linaro.org>
Message-id: 20200228153619.9906-15-peter.maydell@linaro.org