Yea, it is a shame on Barcelona to finish the game with just one goal, even with shut one that their opponent had. However, as long as they are able to have 3 points, it is a good result.
I believe that they know what they are doing, and Xavi is only after securing three points no matter how the goal was scored and how many goals. He might be reserving the players for the champions league since he knows that he cannot win Laliga but might want to make sure that his players go far in the champions league.
I am not sure that this is true. I think that Xavi would like his team to play great football and gain confidence for the rest of the season. Playing dominant games really helps to boost confidence. The Champions League games will soon take place and a performance like that against Las Palmas would never be sufficient to win the games against PSG.
Xavi has had his team play a minimalistic style for so long this season that nobody is satisfied with what they are doing anymore despite winning the games. Barcelona isn't a result-oriented team. They are known for the best football to be played in that sport but those times are over.