vdpa/mlx5: Small improvement for change_num_qps()

change_num_qps() has a lot of multiplications by 2 to convert
the number of VQ pairs to number of VQs. This patch simplifies
the code by doing the VQP -> VQ count conversion at the beginning
in a variable.

Signed-off-by: Dragos Tatulea <dtatulea@nvidia.com>
Reviewed-by: Tariq Toukan <tariqt@nvidia.com>
Message-Id: <20240816090159.1967650-10-dtatulea@nvidia.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Eugenio Pérez <eperezma@redhat.com>
Tested-by: Lei Yang <leiyang@redhat.com>