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>

Author: sean-jc

arch/x86/include/asm/kvm_host.h

@@ -1410,8 +1410,6 @@ struct kvm_arch {
 	struct delayed_work kvmclock_update_work;
 	struct delayed_work kvmclock_sync_work;
 
-	struct kvm_xen_hvm_config xen_hvm_config;
-
 	/* reads protected by irq_srcu, writes by irq_lock */
 	struct hlist_head mask_notifier_list;
 
@@ -1421,6 +1419,7 @@ struct kvm_arch {
 
 #ifdef CONFIG_KVM_XEN
 	struct kvm_xen xen;
+	struct kvm_xen_hvm_config xen_hvm_config;
 #endif
 
 	bool backwards_tsc_observed;