vmd: Use bus_generic_rman_* for PCI bus and memory resources

While here, add custom bus_map/unmap_resource methods to request
mappings via the window memory resources allocated from the parent
bus.

Tested by: emaste
Differential Revision: https://reviews.freebsd.org/D43886

(cherry picked from commit d714e73f789515963a22fe64417bf3883cdb599c)