Move Xen binding / ioctl definitions to xen-sys crate #347
Description
Currently in mmap/xen.rs we have Xen bindings, which shouldn't live in vm-memory. They should be moved to xen-sys. I think originally this was not done because it wasnt clear how to do Xen tests in CI for xen-sys, but having the bindings in vm-memory instead does not solve that problem at all (because we also dont have Xen CI here), so let's just move them.
These things are not publicly exported, so this should not be a breaking change.