vdpa/mlx5: Start off rqt_size with max VQPs

Currently rqt_size is initialized during device flag configuration.
That's because it is the earliest moment when device knows if MQ
(multi queue) is on or off.

Shift this configuration earlier to device creation time. This implies
that non-MQ devices will have a larger RQT size. But the configuration
will still be correct.

This is done in preparation for the pre-creation of hardware virtqueues
at device add time. When that change will be added, RQT will be created
at device creation time so it needs to be initialized to its max size.

Reviewed-by: Cosmin Ratiu <cratiu@nvidia.com>
Acked-by: Eugenio Pérez <eperezma@redhat.com>
Signed-off-by: Dragos Tatulea <dtatulea@nvidia.com>
Message-Id: <20240626-stage-vdpa-vq-precreate-v2-13-560c491078df@nvidia.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>