Commit 6a0b7505 which added documentation of the virt board
crossed in the post with commit 6f4e1405 which added a new
'mte' machine option. Update the docs to include the new option.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>