s390: kvm: Cpu model support for msa6, msa7 and msa8

msa6 and msa7 require no changes.
msa8 adds kma instruction and feature area.

Signed-off-by: Jason J. Herne <jjherne@linux.vnet.ibm.com>
Reviewed-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>