Add function caller_sites verification in python api

[SmoothHacker]

Fixes #3722

[psifertex]

Thanks for the PR! It's a good idea however there's actually another approach we might need to implement but I'd have to double check the implementation. I believe the correct approach is to actually walk the IL in the first place of each caller_function and first check if it's an LLIL call and then return it. I'd have to dig up a sample that replicates it though to verify whether that is a better approach or not. Either way there's no reason not to at least add this.

Do you mind renaming x to ref though to be a bit more explicit? Super minor, sorry.

[SmoothHacker]

No worries, I was tired when I made this PR