KVM: x86/xen: Remove unneeded xen context from kvm_arch when !CONFIG_KVM_XEN

KVM: x86/xen: Remove unneeded xen context from kvm_arch when !CONFIG_KVM_XEN

Saving a few bytes of memory per KVM VM is certainly great but what's more
important is the ability to see where the code accesses Xen emulation
context while CONFIG_KVM_XEN is not enabled. Currently, kvm_cpu_get_extint()
is the only such place and it is harmless: kvm_xen_has_interrupt() always
returns '0' when !CONFIG_KVM_XEN.

No functional change intended.

Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>
Tested-by: Jeremi Piotrowski <jpiotrowski@linux.microsoft.com>
Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Link: https://lore.kernel.org/r/20231205103630.1391318-2-vkuznets@redhat.com
Signed-off-by: Sean Christopherson <seanjc@google.com>