virHostdevPrepareSCSIVHostDevices: Simplify logic

virHostdevPrepareSCSIVHostDevices: Simplify logic

Couple of things happening in this patch:

1) We can mark the device we're adding onto active list as used
   way before - when adding it onto temporary list.

2) When actually moving device from a temporary helper list onto
   the list of active devices we check if the device isn't
   already there. The same check is performed by
   virSCSIVHostDeviceListAdd() later. Drop this duplicity.

3) The 'error' label is renamed to 'rollback' to reflect what it
   is actually doing. While in the rest of the code we don't
   allow random label names, this source file is different.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>