kvm: nVMX: small cleanup in handle_exception

The reason for skipping handling of NMI and #MC in handle_exception is
the same, namely they are handled earlier by vmx_complete_atomic_exit.
Calling the machine check handler (which just returns 1) is misleading,
don't do it.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>