add ibpb, ssbd, virt-ssbd, amd-ssbd, amd-no-ssb, pdpe1gb cpu flags

add ibpb, ssbd, virt-ssbd, amd-ssbd, amd-no-ssb, pdpe1gb cpu flags

> The following are important CPU features that should be used on
> Intel x86 hosts, when available in the host CPU. Some of them
> require explicit configuration to enable, as they are not included
> by default in some, or all, of the named CPU models listed above.
> In general all of these features are included if using “Host
> passthrough” or “Host model”.
>
> pcid: Recommended to mitigate the cost of the Meltdown
> (CVE-2017-5754) fix. Included by default in Haswell, Broadwell &
> Skylake Intel CPU models. Should be explicitly turned on for
> Westmere, SandyBridge, and IvyBridge Intel CPU models. Note that
> some desktop/mobile Westmere CPUs cannot support this feature.
>
> spec-ctrl: Required to enable the Spectre (CVE-2017-5753 and
> CVE-2017-5715) fix, in cases where retpolines are not sufficient.
> Included by default in Intel CPU models with -IBRS suffix. Must be
> explicitly turned on for Intel CPU models without -IBRS suffix.
> Requires the host CPU microcode to support this feature before it
> can be used for guest CPUs.
>
> ssbd: Required to enable the CVE-2018-3639 fix. Not included by
> default in any Intel CPU model. Must be explicitly turned on for
> all Intel CPU models. Requires the host CPU microcode to support
> this feature before it can be used for guest CPUs.
>
> pdpe1gbr: Recommended to allow guest OS to use 1GB size pages.Not
> included by default in any Intel CPU model. Should be explicitly
> turned on for all Intel CPU models. Note that not all CPU hardware
> will support this feature.
-- https://www.berrange.com/posts/2018/06/29/cpu-model-configuration-for-qemu-kvm-on-x86-hosts/