Document the arm 'virt' board, which has been undocumented
for far too long given that it is the main recommended board
type for arm guests.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Message-id: 20200713175746.5936-5-peter.maydell@linaro.org