kvm: x86: Move last_cpu into kvm_vcpu_arch as last_vmentry_cpu

kvm: x86: Move last_cpu into kvm_vcpu_arch as last_vmentry_cpu

Both the vcpu_vmx structure and the vcpu_svm structure have a
'last_cpu' field. Move the common field into the kvm_vcpu_arch
structure. For clarity, rename it to 'last_vmentry_cpu.'

Suggested-by: Sean Christopherson <[email protected]>
Signed-off-by: Jim Mattson <[email protected]>
Reviewed-by: Oliver Upton <[email protected]>
Reviewed-by: Peter Shier <[email protected]>
Message-Id: <20200603235623[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>