vhost-user: Correct a reference of TARGET_AARCH64

vhost-user: Correct a reference of TARGET_AARCH64

Presumably TARGET_ARM_64 should be a mistake of TARGET_AARCH64.

Signed-off-by: Akihiko Odaki <akihiko.odaki@daynix.com>
Message-Id: <20230109063130.81296-1-akihiko.odaki@daynix.com>
Fixes: 27598393a2 ("Lift max memory slots limit imposed by vhost-user")
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>