meson, xen: fix condition for enabling the Xen accelerator

meson, xen: fix condition for enabling the Xen accelerator

A misspelled condition in xen_native.h is hiding a bug in the enablement of
Xen for qemu-system-aarch64.  The bug becomes apparent when building for
Xen 4.18.

While the i386 emulator provides the xenpv machine type for multiple architectures,
and therefore can be compiled with Xen enabled even when the host is Arm, the
opposite is not true: qemu-system-aarch64 can only be compiled with Xen support
enabled when the host is Arm.

Expand the computation of accelerator_targets['CONFIG_XEN'] similar to what is
already there for KVM.

Cc: Stefano Stabellini <stefano.stabellini@amd.com>
Cc: Richard W.M. Jones <rjones@redhat.com>
Cc: Daniel P. Berrangé <berrange@redhat.com>
Reported-by: Michael Young <m.a.young@durham.ac.uk>
Fixes: 0c8ab1cddd6 ("xen_arm: Create virtio-mmio devices during initialization", 2023-08-30)
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>