KVM: VMX: Move nmi injection failure processing to vm exit path

Instead of processing nmi injection failure in the vm entry path, move
it to the vm exit path (vm_complete_interrupts()).  This separates nmi
injection from nmi post-processing, and moves the nmi state from the VT
state into vcpu state (new variable nmi_injected specifying an injection
in progress).

Signed-off-by: Avi Kivity <avi@qumranet.com>