This patch adds the implementation of the SBBC pSeries feature,
using the QEMU_CAPS_MACHINE_PSERIES_CAP_SBBC capability added
in the previous patch.

Like the previously added CFPC feature, SBBC can have the values
"broken", "workaround" or "fixed". Extra code is required to handle
it since it's not a regular tristate capability.

This is the XML format for the cap:

<features>
  <sbbc value='workaround'/>
</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>