KVM: x86/xen: Consult kvm_xen_enabled when checking for Xen MSR writes

Query kvm_xen_enabled when detecting writes to the Xen hypercall page MSR
so that the check is optimized away in the likely scenario that Xen isn't
enabled for the VM.

Deliberately open code the check instead of using kvm_xen_msr_enabled() in
order to avoid a double load of xen_hvm_config.msr (which is admittedly
rather pointless given the widespread lack of READ_ONCE() usage on the
plethora of vCPU-scoped accesses to kvm->arch.xen state).

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/20250215011437.1203084-4-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

arch/x86/kvm/xen.h

@@ -58,6 +58,9 @@ static inline bool kvm_xen_msr_enabled(struct kvm *kvm)
 
 static inline bool kvm_xen_is_hypercall_page_msr(struct kvm *kvm, u32 msr)
 {
+	if (!static_branch_unlikely(&kvm_xen_enabled.key))
+		return false;
+
 	return msr && msr == kvm->arch.xen_hvm_config.msr;
 }