hw/ppc/spapr: Implement h_set_dabr

According to LoPAPR, h_set_dabr should simply set DABRX to 3
(if the register is available), and load the parameter into DABR.
If DABRX is not available, the hypervisor has to check the
"Breakpoint Translation" bit of the DABR register first.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
