qdev-properties.h: Explicitly set the default value for arraylen properties

In DEFINE_PROP_ARRAY, because we use a PropertyInfo (qdev_prop_arraylen)
which has a .set_default_value member we will set the field to a default
value. That default value will be zero, by the C rule that struct
initialization sets unmentioned members to zero if at least one member
is initialized. However it's clearer to state it explicitly.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-id: 1499788408-10096-2-git-send-email-peter.maydell@linaro.org
