x86/spec-ctrl: Wire up the Native-BHI software sequences

x86/spec-ctrl: Wire up the Native-BHI software sequences

In the absence of BHI_DIS_S, mitigating Native-BHI requires the use of a
software sequence.

Introduce a new bhb-seq= option to select between avaialble sequences and
bhb-entry= to control the per-PV/HVM actions like we have for other blocks.

Activate the short sequence by default for PV and HVM guests on affected
hardware if BHI_DIS_S isn't present.

This is part of XSA-456 / CVE-2024-2201.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Roger Pau Monné <roger.pau@citrix.com>