KVM: SVM: Implement check_nested_events for NMI

Migrate nested guest NMI intercept processing
to new check_nested_events.

Signed-off-by: Cathy Avery <cavery@redhat.com>
Message-Id: <20200414201107.22952-2-cavery@redhat.com>
[Reorder clauses as NMIs have higher priority than IRQs; inject
 immediate vmexit as is now done for IRQ vmexits. - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>