For partition mode, there is no vm0_domain created for iommu.
In current code, it will return error when try to remove a device
from vm0_domain, which casue failure of device assignment in
partition mode.
Tracked-On: #1680
Signed-off-by: Binbin Wu <binbin.wu@intel.com>
Acked-by: Eddie Dong <eddie.dong@intel.com>