KVM: s390: introduce KMSG_COMPONENT for kvm-s390

Let's remove "kvm-s390" from our printk messages and make use
of pr_fmt instead.

Also replace one printk() occurrence by a equivalent pr_warn
on the way.

Suggested-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: David Hildenbrand <dahi@linux.vnet.ibm.com>
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>