KVM: x86: Add SBPB support

Add support for the AMD Selective Branch Predictor Barrier (SBPB) by
advertising the CPUID bit and handling PRED_CMD writes accordingly.

Note, like SRSO_NO and IBPB_BRTYPE before it, advertise support for SBPB
even if it's not enumerated by in the raw CPUID.  Some CPUs that gained
support via a uCode patch don't report SBPB via CPUID (the kernel forces
the flag).

Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
Link: https://lore.kernel.org/r/a4ab1e7fe50096d50fde33e739ed2da40b41ea6a.1692919072.git.jpoimboe@kernel.org


Co-developed-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>