Macrofy the definition of kvm_exit so that the definition can be reused
verbatim by kvm_nested_vmexit.

No functional change intended.

Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Message-Id: <20200923201349.16097-6-sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>