KVM: PPC: Book3S: Ignore DABR register

We don't emulate breakpoints yet, so just ignore reads and writes
to / from DABR.

This fixes booting of more recent Linux guest kernels for me.

Reported-by: Nello Martuscielli <ppc.addon@gmail.com>
Tested-by: Nello Martuscielli <ppc.addon@gmail.com>
Signed-off-by: Alexander Graf <agraf@suse.de>