storage: Don't update volume objs list before we successfully create one

storage: Don't update volume objs list before we successfully create one

We do update pool volume object list before we actually create any
volume. If buildVol fails, we then try to delete the volume in the
storage as well as remove it from our structures. The problem is, that
any backend that supports both buildVol and deleteVol would fail in this
case which is completely unnecessary. This patch causes the update to
take place after we know a volume has been created successfully, thus no
removal in case of a buildVol failure is necessary.

https://bugzilla.redhat.com/show_bug.cgi?id=1223177