Both the existing darwin backend and the upcoming windows backend
drive USB devices from user space (IOUSBHost CGO calls vs. VBoxUSB
IOCTLs). Refactor the per-attachment URB loop out of host_darwin.go
into a platform-agnostic userspaceURBSession that talks to a URBEngine
interface; the darwin-specific dispatch becomes a 30-line
darwinIOUSBHostEngine. Linux's kernelHandoffSession is untouched.
Move hex8 into shared.go and add usbipStatusEIO so the shared session
does not depend on golang.org/x/sys/unix (Windows has no equivalent).
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