intel-iommu: Fix reattaching of devices to identity mapping domain

When we reattach a device to the si_domain (because it's been removed
from a VM), we weren't calling domain_context_mapping() to actually tell
the hardware about that.

We should really put the call to domain_context_mapping() into
domain_add_dev_info() -- we never call the latter without also doing the
former, and we can keep the error paths simple that way. But that's a
cleanup which can wait for 2.6.32 now.

Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>