vt-d: Do FLR of assigned devices with VT-d

vt-d: Do FLR of assigned devices with VT-d

Currently there is a pdev_flr() function to do FLR before device
assignment in qemu, but most of devices don't have FLR capability.
What's more, should do FLR before assignment and deassignment for
keeping correct device status. If the device doesn't have FLR
capablility, this patch implemented to enter D3hot and return to D0 to
do FLR. And exposed pdev_flr() in VT-d utils, then it can be invoked
by assignment and deassignment functions.

Signed-off-by: Weidong Han <weidong.han@intel.com>
Signed-off-by: Anthony Xu <anthony.xu@intel.com>