Enable HVM guest VT-d device hotplug via a simple ACPI hotplug device model.

Enable HVM guest VT-d device hotplug via a simple ACPI hotplug device model.

** Currently only 2 virtual hotplug pci slots(6~7) are created so more
   than 2 vtd dev can't be hotplugged, but we can easily extend it in
   future.

Three new commands are added:
"xm pci-list domid" show the current assigned vtd device, like:
VSlt  domain   bus   slot   func
0x6      0x0  0x02   0x00    0x0

"xm pci-detach" hot remove the specified vtd device by the virtual
slot, like:
xm pci-detach EdwinHVMDomainVtd 6

"xm pci-attach DomainID dom bus dev func [vslot]" hot add a new vtd
device in the vslot. If no vslot specified, a free slot will be picked
up. e.g. to insert '0000:03:00.0':
xm pci-attach EdwinHVMDomainVtd 0 3 0 0

** guest pci hotplug
linux: pls. use 2.6.X and enable ACPI PCI hotplug ( Bus options=> PCI
hotplug => ACPI PCI hotplug driver )
windows: 2000/xp/2003/vista are all okay

Signed-off-by: Zhai Edwin <edwin.zhai@intel.com>