This patch adds the implementation of the IBS pSeries feature,
using the QEMU_CAPS_MACHINE_PSERIES_CAP_IBS capability added
in the previous patch.

IBS can have the following values: "broken", "workaround",
"fixed-ibs", "fixed-ccd" and "fixed-na".

This is the XML format for the cap:

<features>
  <ibs value='fixed-ibs'/>
</features>
Signed-off-by: NDaniel Henrique Barboza <danielhb413@gmail.com>
Signed-off-by: NMichal Privoznik <mprivozn@redhat.com>
Reviewed-by: NMichal Privoznik <mprivozn@redhat.com>