KVM: s390: Allow NULL parameter for kvm_s390_get_regs_rre

We're not always interested in both registers that are specified
for an RRE instruction. So allow NULL as parameter, too, to indicate
that we do not need the corresponding value.

Signed-off-by: Thomas Huth <thuth@linux.vnet.ibm.com>
Reviewed-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>