KVM: ppc: Add dbsr in kvm_vcpu_arch

Kernel for E500 need clear dbsr when startup.
So add dbsr register in kvm_vcpu_arch for BOOKE.

Signed-off-by: Liu Yu <yu.liu@freescale.com>
Signed-off-by: Hollis Blanchard <hollisb@us.ibm.com>
Signed-off-by: Avi Kivity <avi@redhat.com>