usb: separate generic USB layer from xHCI implementation (prepare for non-xHCI controllers)

[emilf]

Today kernel/src/drivers/usb/mod.rs is mostly a thin re-export layer around the xHCI driver (e.g. service_runtime_interrupt, kick_msix_self_test, diagnostics APIs). The interrupt handler handler_usb_xhci calls into crate::drivers::usb::service_runtime_interrupt(), which currently just forwards to xHCI.

This makes it hard to support other host controller types (EHCI/OHCI/UHCI, or future controllers) and also complicates running under hypervisors/firmware setups that may expose older controllers by default.

Proposal:

• Introduce a generic USB host-controller abstraction layer (HCD trait) and a USB core module that owns higher-level runtime servicing APIs.
• xHCI becomes one HCD implementation; other controllers can be added later or stubbed.
• Move xHCI-specific diagnostics/self-tests under an xHCI namespace; keep generic hooks in drivers::usb.

Acceptance:

• drivers::usb provides controller-agnostic init + runtime servicing hooks.
• Interrupt handlers call into the generic USB layer, which dispatches to registered HCDs.