hw/xen/hvm: Fix Aarch64 typo

hw/xen/hvm: Fix Aarch64 typo

There is no TARGET_ARM_64 definition. Luckily enough,
when TARGET_AARCH64 is defined, TARGET_ARM also is.

Fixes: 733766cd373 ("hw/arm: introduce xenpvh machine")
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20250305153929.43687-2-philmd@linaro.org>
(cherry picked from commit 3a11b653a63fee0e43f4ab84b93f068b961d8fe7)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>