KVM: s390: guestdbg: separate defines for per code

Let's avoid working with the PER_EVENT* defines, used for control register
manipulation, when checking the u8 PER code. Introduce separate defines
based on the existing defines.

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