support live migration with direct pci passthrough

SR-IOV has been supported for a long while, in the community's point of view,
the pci passthru with Macvtap can be live migrated possibly, but the direct pci passthru
seems hard to implement the migration as the passthru VF is totally controlled by
the VMs so that some internal states may be unknown by the hypervisor.

But we think the direct pci passthru model can also be live migrated with the
following combination of a series of technology/operation based on the enhanced
Qemu-Geust-Agent(QGA) which has already been supported by nova.
   1)Bond the direct pci passthru NIC with a virtual NIC.
     This will keep the network connectivity during the live migration.
   2)Unenslave the direct pci passthru NIC
   3)Hot-unplug the direct pci passthru NIC
   4)Live-migrate guest with the virtual NIC
   5)Hot-plug the direct pci passthru NIC on the target host
   6)Enslave the direct pci passthru NIC
More detailed thoughts about the 6 steps above please refer to the following URL:
http://paste.openstack.org/show/487066/

And more inforation about this concept can refer to [1].
[1]https://www.kernel.org/doc/ols/2008/ols2008v2-pages-261-267.pdf