KVM: nSVM: Add helper to synthesize nested VM-Exit without collateral

Add a helper to consolidate boilerplate for nested VM-Exits that don't
provide any data in exit_info_*.

No functional change intended.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20210302174515.2812275-3-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>