KVM: Move NMI IRET fault processing to new vmx_complete_interrupts()

Currently most interrupt exit processing is handled on the entry path,
which is confusing.  Move the NMI IRET fault processing to a new function,
vmx_complete_interrupts(), which is called on the vmexit path.

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