KVM: x86/xen: Add an #ifdef'd helper to detect writes to Xen MSR

KVM: x86/xen: Add an #ifdef'd helper to detect writes to Xen MSR

Add a helper to detect writes to the Xen hypercall page MSR, and provide a
stub for CONFIG_KVM_XEN=n to optimize out the check for kernels built
without Xen support.

Reviewed-by: Paul Durrant <paul@xen.org>
Reviewed-by: David Woodhouse <dwmw@amazon.co.uk>
Link: https://lore.kernel.org/r/20250215011437.1203084-3-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>