MFC 305502: Reset PCI pass through devices via PCI-e FLR during VM start/end.

MFC 305502: Reset PCI pass through devices via PCI-e FLR during VM start/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.

Sponsored by:	Chelsio Communications

git-svn-id: svn://svn.freebsd.org/base/stable/10@306520 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f