KVM: x86/xen: Bury xen_hvm_config behind CONFIG_KVM_XEN=y

Now that all references to kvm_vcpu_arch.xen_hvm_config are wrapped with
CONFIG_KVM_XEN #ifdefs, bury the field itself behind CONFIG_KVM_XEN=y.

No functional change intended.

Reviewed-by: David Woodhouse <dwmw@amazon.co.uk>
Reviewed-by: Paul Durrant <paul@xen.org>
Link: https://lore.kernel.org/r/20250215011437.1203084-5-seanjc@google.com


Signed-off-by: Sean Christopherson <seanjc@google.com>