iommu/vt-d: Check the return value of iommu_device_create()

This adds the proper check to alloc_iommu to make sure that
the call to iommu_device_create has completed successfully
and if not return the error code to the caller after freeing
up resources allocated previously.

Signed-off-by: Nicholas Krause <xerofoify@gmail.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>