The rivets are just plastic and aluminium. If there are any left in place, you can drill them out with a small diameter drill-bit. (I think you need one about 1/8th of an inch or so. This bores out the rivet portion, and the two halves just fall off. Just be careful not to enlarge the holes in the liner & bumper in which the rivets are installed!

You can get replacement rivets from a Volvo dealer or most major hardware stores. They are easy to install with a hand rivet tool (also available at a major hardware store for about $20). Just push the new rivet post through the holes in the liner and bumper, and squeeze it tight with the rivet tool. The river tool will pull the back portion of the rivet post towards the front, crushing a flexible plastic portion, and forming the rivet holding the two pieces together.

It's very easy to work with. You might find it helps to remove the front tires to provide better access, but you can do it with the tires turned to full-stop.