vt-d: Only setup rmrr identity mappings of the assigned devices

vt-d: Only setup rmrr identity mappings of the assigned devices

Current implementation setups identity mappings for all the rmrrs
whether the assigned device has rmrr or not. It's wasteful and not
necessary, because only USB controllers and UMA integarated graphics
devices have rmrr. This patch only setups rmrr identity mappings of
the assigned devices. If the assigned devices don't have rmrr, needn't
to setup rmrr identity mapping for guest.

BTW, this patch fixes the VT-d bug (Fail to boot smp Linux guest with
VT-d NIC assigned on IA32e platform).

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