KVM: x86: Route pending NMIs from userspace through process_nmi()

Use the asynchronous NMI queue to handle pending NMIs coming in from
userspace during KVM_SET_VCPU_EVENTS so that all of KVM's logic for
handling multiple NMIs goes through process_nmi().  This will simplify
supporting SVM's upcoming "virtual NMI" functionality, which will need
changes KVM manages pending NMIs.

Signed-off-by: Sean Christopherson <seanjc@google.com>