KVM: s390: protvirt: Add KVM api documentation

Add documentation for KVM_CAP_S390_PROTECTED capability and the
KVM_S390_PV_COMMAND ioctl.

Signed-off-by: Janosch Frank <frankja@linux.ibm.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
[borntraeger@de.ibm.com: patch merging, splitting, fixing]
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>