Add -cpu a64fx to use A64FX processor when -machine virt option is
specified.  In addition, add a64fx to the Supported guest CPU types
in the virt.rst document.

Signed-off-by: Shuuichirou Ishii <ishii.shuuichir@fujitsu.com>
Reviewed-by: Andrew Jones <drjones@redhat.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>