Fix typo in xhci nvlist node name, and also increment device counter.

This allows the xhci tablet device to be recognized and a PCI device
instantiated.

Reviewed by: jhb
Fixes: 621b5090487d Refactor configuration management in bhyve.
MFC after: 3 months.

(cherry picked from commit ab899f8937c1c7c79197baf7192b727ddc9cfe54)