s390x: stsi: Define vm_is_kvm to be used in different tests

Several tests are in need of a way to check on which hypervisor
and virtualization level they are running on to be able to fence
certain tests. This patch adds functions that return true if a
vm is running under KVM, LPAR or generally as a level 2 guest.

To check if we're running under KVM we use the STSI 3.2.2
instruction, let's define it's response structure in a central
header.

Signed-off-by: Pierre Morel <pmorel@linux.ibm.com>
Reviewed-by: Janosch Frank <frankja@linux.ibm.com>
Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com>