lib: s390x: sie: Set guest memory pointer

Seems like it was introduced but never set. It's nicer to have a
pointer than to cast the MSO of a VM.

Signed-off-by: Janosch Frank <frankja@linux.ibm.com>
Reviewed-by: Nico Boehr <nrb@linux.ibm.com>
Reviewed-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Link: https://lore.kernel.org/r/20230112154548.163021-6-frankja@linux.ibm.com
Message-Id: <20230112154548.163021-6-frankja@linux.ibm.com>