bhyve: Populate the device version from the backend
The pci_xhci driver requires the USB device version to be known before
allocating a hub port. To support this, we split the original xHCI
initialization into two phases:
- Probe: Parse the nvlist and determine the device version.
- Init: Complete initialization and set up the softc details.
This change ensures proper hub port allocation based on accurate device
version.
Approved by: markj (mentor)
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D51478