VT-d: Fix ATS enabling for device assignment

VT-d: Fix ATS enabling for device assignment

Currently, Xen only enables ATS in Xen booting. When an ATS capable
device is assigned to guest, ATS is actually not enabled because FLR
before assignment causes it to be disabled. Thus ATS cannot be used in
guest. This patch enables ATS in domain_context_mapping. This ensures
ATS is enabled in assignment because FLR is earlier than
domain_context_mapping call. Therefore ATS can be used in guest. This
patch also implements disable_ats_device to disable ATS when the
device is deassigned from a domain.

Signed-off-by: Weidong Han <weidong.han@intel.com>