VT-d: fix device assignment failure (regression from Xen c/s 19805:2f1fa2215e60)

VT-d: fix device assignment failure (regression from Xen c/s 19805:2f1fa2215e60)

If the device at <secbus>:00.0 is the device the mapping operation was
initiated for, trying to map it a second time will fail, and hence
this second mapping attempt must be prevented (as was done prior to
said c/s).

While at it, simplify the code a little, too.

Signed-off-by: Jan Beulich <jbeulich@novell.com>
Acked-by: Weidong Han <weidong.han@intel.com>