hw/arm/virt: Add "max" to the list of CPU types "virt" supports

Allow the virt board to support '-cpu max' in the same way
it already handles '-cpu host'.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 20180308130626.12393-6-peter.maydell@linaro.org
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>