bhyve: improve input validation in pci_xhci

Several functions did not validate the slot index resulting in OOB read
on the heap of the slot device structure which could lead to arbitrary
reads/writes and potentially code execution.

Reported by: Synacktiv
Reviewed by: markj (earlier), jhb
Security: CVE-2024-41721
Security: HYP-02
Sponsored by: The Alpha-Omega Project
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D45996