vdpa/mlx5: Initialize and reset device with one queue pair

The virtio spec says that a vdpa device should start off with one queue
pair. The driver is already compliant.

This patch moves the initialization to device add and reset times. This
is done in preparation for the pre-creation of hardware virtqueues at
device add time.

Signed-off-by: Dragos Tatulea <dtatulea@nvidia.com>
Reviewed-by: Cosmin Ratiu <cratiu@nvidia.com>
Message-Id: <20240626-stage-vdpa-vq-precreate-v2-7-560c491078df@nvidia.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Eugenio Pérez <eperezma@redhat.com>