IBS (Indirect Branch Speculation) is the last capability added
in QEMU 2.12 related to Spectre mitigation for Power. It was
added in commit 4be8d4e7d935.

This patch introduces it as QEMU_CAPS_MACHINE_PSERIES_CAP_IBS.
Like CFPC and SBBC, users might want to tune in IBS based on
their HW and guest OS requirements, and it's better to do it
so in a proper Libvirt feature than to put QEMU arguments
in the middle of the domain XML.
Signed-off-by: NDaniel Henrique Barboza <danielhb413@gmail.com>
Reviewed-by: NMichal Privoznik <mprivozn@redhat.com>