KVM: nVMX: vmx_complete_nested_posted_interrupt() can't fail

(cherry picked from commit 6342c50ad12e8ce0736e722184a7dbdea4a3477f)

vmx_complete_nested_posted_interrupt() can't fail, let's turn it into
a void function.

Signed-off-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>