cam/ata: Allow drivers to veto ATA disk attachment.

This eventhandler is mainly used by VMs, e.g. Hyper-V, whose disk
controllers share the disks with the simulated ATA controllers.

Submitted by: Hongjiang Zhang <honzhan microsoft com>
Discussed with: mav
MFC after: 1 week
Sponsored by: Microsoft
Differential Revision: https://reviews.freebsd.org/D7693