VT-d: fix invalid domain id for KVM context flush

The domain->id is a sequence number associated with the KVM guest
and should not be used for the context flush. This patch replaces
the domain->id with a proper id value for both bare metal and KVM.

Signed-off-by: Yu Zhao <yu.zhao@intel.com>
Acked-by: Weidong Han <weidong.han@intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>