vmd: Rename vmd_bus class to pci

This fixes a kernel panic when probing for vmd_bus on Intel TigerLake on
14-CURRENT. Apparently, vmd_bus is a type of PCI bus, but was registered
as a separate device class.

PR: 256915
Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D31071

(cherry picked from commit ad1f608fb2f529baf028384bbe7e8fbbff5cbe23)