meson.build: enable xenpv machine build for ARM

Add CONFIG_XEN for aarch64 device to support build for ARM targets.

Signed-off-by: Vikram Garhwal <vikram.garhwal@amd.com>
Signed-off-by: Stefano Stabellini <stefano.stabellini@amd.com>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>