Reset PCI pass through devices via PCI-e FLR during VM start and end.

Add routines to trigger a function level reset (FLR) of a PCI-express
device via the PCI-express device control register. This also includes
support routines to wait for pending transactions to complete as well
as calculating the maximum completion timeout permitted by a device.

Change the ppt(4) driver to reset pass through devices before attaching
to a VM during startup and before detaching from a VM during shutdown.

Reviewed by: imp, wblock (earlier version)
MFC after: 1 month
Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D7751