nodedev: improve error message when destroying an inactive device

nodedev: improve error message when destroying an inactive device

When trying to destroy a node device that is not active, we end up with
a confusing error message:

  # nodedev-destroy mdev_88a6b868_46bd_4015_8e5b_26107f82da38
  error: Failed to destroy node device 'mdev_88a6b868_46bd_4015_8e5b_26107f82da38'
  error: failed to access '/sys/bus/mdev/devices/88a6b868-46bd-4015-8e5b-26107f82da38/iommu_group': No such file or directory

With this patch, the error is more clear:

  # nodedev-destroy mdev_88a6b868_46bd_4015_8e5b_26107f82da38
  error: Failed to destroy node device 'mdev_88a6b868_46bd_4015_8e5b_26107f82da38'
  error: Requested operation is not valid: Device 'mdev_88a6b868_46bd_4015_8e5b_26107f82da38' is not active

Signed-off-by: Jonathon Jongsma <jjongsma@redhat.com>
Reviewed-by: Boris Fiuczynski <fiuczy@linux.ibm.com>