qapi/machine.json: Escape a literal '*' in doc comment

For rST, '*' is a kind of inline markup (for emphasis), so
"*-softmmu" is a syntax error because of the missing closing '*'.
Escape the '*' with a '\'.

The texinfo document generator will leave the '\' in the
output, which is not ideal, but that generator is going to
go away in a subsequent commit.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-Id: <20200925162316.21205-7-peter.maydell@linaro.org>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>