KVM: X86: Reduce size of kvm_vcpu_arch structure when CONFIG_KVM_XEN=n

When CONFIG_KVM_XEN=n, the size of kvm_vcpu_arch can be reduced
from 5100+ to 4400+ by adding macro control.

Signed-off-by: Peng Hao <flyingpeng@tencent.com>
Link: https://lore.kernel.org/all/CAPm50aKwbZGeXPK5uig18Br8CF1hOS71CE2j_dLX+ub7oJdpGg@mail.gmail.com
[sean: fix whitespace damage]
Signed-off-by: Sean Christopherson <seanjc@google.com>