vtd: Don't do FLR when we create domain or hotplug device into guest.

vtd: Don't do FLR when we create domain or hotplug device into guest.

VT-d hot-plug is done in such a way:  1) xend sends 'pci-ins' command
to ioemu via xenstore; 2) ioemu creates pass-through device and raises
SCI to guest OS; guest OS loads device driver, and device driver
initializes the hot-plugged device; 3) xend does FLR. There is a race
condition about 2) and 3). If 3) runs slow, we have troubles. Thanks
for Yuji Shimada's locating the issues.

We can work around the issues by not doing FLR when we create domain
or hotplug device into guest, namely, we only do FLR when we destroy
domain or hotplug device from guest.

Signed-off-by: Dexuan Cui <dexuan.cui@intel.com>
Signed-off-by: Dongxiao Xu <dongxiao.xu@intel.com>