Allow passthrough devices to be hinted.

Allow the ppt driver to attach to devices that were hinted to be
passthrough devices by the PCI code creating them with a driver
name of "ppt".

Add a tunable that allows the IOMMU to be forced to be used. With
SR-IOV passthrough devices the VFs may be created after vmm.ko is
loaded. The current code will not initialize the IOMMU in that
case, meaning that the passthrough devices can't actually be used.

Differential Revision: https://reviews.freebsd.org/D73
Reviewed by: neel
MFC after: 1 month
Sponsored by: Sandvine Inc.