bhyve: simplify slot validation in xHCI emulation

This is a follow-up to commit e72d86ad9c62 ("bhyve: improve input
validation in pci_xhci") -- introducing a helper for slot validation.

Co-authored-by: John Baldwin <jhb@FreeBSD.org>
Reviewed by: markj, emaste
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D46696

(cherry picked from commit f505f9a842eb3e75e16e6c7c6f67d5b592b1bc65)