KVM: s390: Use the read_guest_abs() in guest debug functions

The guest debug functions work on absolute addresses and should use the
read_guest_abs() function rather than general read_guest() that
works with logical addresses.

Cc: David Hildenbrand <dahi@linux.vnet.ibm.com>
Signed-off-by: Alexander Yarygin <yarygin@linux.vnet.ibm.com>
Reviewed-by: David Hildenbrand <dahi@linux.vnet.ibm.com>
Reviewed-by: Thomas Huth <thuth@linux.vnet.ibm.com>
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>