kvm: better advice for failed s390x startup

kvm: better advice for failed s390x startup

If KVM_CREATE failed on s390x, we print a hint to enable the switch_amode
kernel parameter. This only applies to old kernels, and only if the
error was -EINVAL. Moreover, with new kernels, the most likely reason
for -EINVAL is that pgstes were not enabled.

Let's update the error message to give a better hint on where things
may need fixing.

Acked-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>