vdpa/mlx5: avoid kick handling during shutdown

vdpa/mlx5: avoid kick handling during shutdown

When Qemu suspends a VM, HW notifier is un-mmapped while vCPU thread may
still be active and write notifier through kick socket.

PMD kick handler thread tries to install HW notifier through client
socket. In such case, it will timeout and slow down device close.

This patch skips HW notifier install if VQ or device in middle of
shutdown.

Signed-off-by: Xueming Li <xuemingl@nvidia.com>
Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>