volser: make splitvol use VolumeId

convert split volume rpc to also use volumeid type

Change-Id: I6b1ed670a0abdc1487daa65b7e136a1370afd5fd
Reviewed-on: http://gerrit.openafs.org/8888
Reviewed-by: Chas Williams - CONTRACTOR <chas@cmf.nrl.navy.mil>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>