vdpa/mlx5: fix live migration termination

vdpa/mlx5: fix live migration termination

There are a lot of per virtq operations in the live migration
handling.

Before the driver support for queue update, when a virtq was not valid,
all the LM handling was terminated.

But now, when the driver supports queue update, the virtq can be invalid
as legal stage.

Skip invalid virtq in LM handling.

Fixes: c47d6e83334e ("vdpa/mlx5: support queue update")

Signed-off-by: Matan Azrad <matan@mellanox.com>
Acked-by: Xueming Li <xuemingl@mellanox.com>
Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>