Create/use virStoragePoolObjAddVol in order to add volumes onto list.

Create/use virStoragePoolObjRemoveVol in order to remove volumes from list.

Create/use virStoragePoolObjGetVolumesCount to get count of volumes on list.

For the storage driver, the logic alters when the volumes.obj list grows
to after we've fetched the volobj. This is an optimization of sorts, but
also doesn't "needlessly" grow the volumes.objs list and then just decr
the count if the virGetStorageVol fails.
Signed-off-by: NJohn Ferlan <jferlan@redhat.com>