KVM: x86: Advertise AMD_IBPB_RET to userspace

KVM: x86: Advertise AMD_IBPB_RET to userspace

This is an inherent feature of IA32_PRED_CMD[0], so it is trivially
virtualizable (as long as IA32_PRED_CMD[0] is virtualized).

Suggested-by: Tom Lendacky <thomas.lendacky@amd.com>
Signed-off-by: Jim Mattson <jmattson@google.com>
Reviewed-by: Tom Lendacky <thomas.lendacky@amd.com>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20241011214353.1625057-4-jmattson@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>