Although the rule also allows 'an' for historical reasons,
'a' is more common because 'h' is pronounced.
Reviewed-by: NPeter Maydell <peter.maydell@linaro.org>
Signed-off-by: NStefan Weil <sw@weilnetz.de>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>