Add pvcreate_params to vg_extend.

Add pvcreate_params to vg_extend.

Another refactoring for implicit pvcreate support.  We need to get
the pvcreate parameters somehow to the vg_extend routine.  Options
seemed to be:
1. Attach the parameters to struct volume_group.  I personally
did not like this idea in most cases, though one could make an
agrument why it might be ok at least for some of the parameters
(e.g. metadatacopies).
2. Pass them in to the extend routine.  This second route seemed
to be the best approach given the constraints.

Future patches will parse the command line and fill in the actual
values for the pvcreate_single call.
Should be no functional change.