KVM: s390: add vector enhancements facility 2 to cpumodel

If vector support is enabled, the vector enhancements facility 2
might also be enabled.
We can directly forward this facility to the guest if available
and VX is requested by user space.

Please note that user space can and will have the final decision
on the facility bits for a guests.

Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Reviewed-by: Janosch Frank <frankja@linux.ibm.com>
Reviewed-by: Collin Walling <walling@linux.ibm.com>
Reviewed-by: David Hildenbrand <david@redhat.com>