KVM: VMX: Rewrite vmx_complete_interrupt()'s twisted maze of if() statements

...with a more straightforward switch().

Also fix a bug when NMI could be dropped on exit. Although this should
never happen in practice, since NMIs can only be injected, never triggered
internally by the guest like exceptions.

Signed-off-by: Gleb Natapov <gleb@redhat.com>
Signed-off-by: Avi Kivity <avi@redhat.com>