XAVC-L is not supported yet in FCPX. You can either use Edit Ready from Divergent Media or Sony Catalyst Browse to transcode to ProRes.
Edit Ready is $50, Catalyst Browse is free. I’ve read good things about Edit Ready, but haven’t tried it. Catalyst Browse is said to introduce a gamma shift in batch encoding but I haven’t seen that in my limited trials.
Edit Ready offers a free trial that limits encoding to the first minute of a clip.
The Sony PXW-X70 records XAVC-L or AVCHD. There are quite a few X70 owners who edit in FCPX and are anxiously awaiting XAVC-L support.
https://www.sonycreativesoftware.com/catalystbrowse
https://www.divergentmedia.com/editready