Update the documentation to match current QEMU.
Remove the mention of pre-2.7 machine models as those aren't provided
anymore.
Signed-off-by: Mohamed Mediouni <mohamed@unpredictable.fr>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>