KVM: change kvm_assign_device() to print return value when iommu_attach_device() fails

Change existing kernel error message to include return value from
iommu_attach_device() when it fails. This will help debug device
assignment failures more effectively.

Signed-off-by: Shuah Khan <shuah.khan@hp.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>