KVM: s390: Fix clock comparator field for STORE STATUS

Only the most 7 significant bytes of the clock comparator must be
saved to the status area, and the byte at offset 304 has to be zero.

Signed-off-by: Thomas Huth <thuth@linux.vnet.ibm.com>
Reviewed-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>