x86/cpu/kvm: Provide UNTRAIN_RET_VM

Similar to how it doesn't make sense to have UNTRAIN_RET have two
untrain calls, it also doesn't make sense for VMEXIT to have an extra
IBPB call.

This cures VMEXIT doing potentially unret+IBPB or double IBPB.
Also, the (SEV) VMEXIT case seems to have been overlooked.

Redefine the meaning of the synthetic IBPB flags to:

 - ENTRY_IBPB     -- issue IBPB on entry  (was: entry + VMEXIT)
 - IBPB_ON_VMEXIT -- issue IBPB on VMEXIT

And have 'retbleed=ibpb' set *BOTH* feature flags to ensure it retains
the previous behaviour and issues IBPB on entry+VMEXIT.

The new 'srso=ibpb_vmexit' option only sets IBPB_ON_VMEXIT.

Create UNTRAIN_RET_VM specifically for the VMEXIT case, and have that
check IBPB_ON_VMEXIT.

All this avoids having the VMEXIT case having to check both ENTRY_IBPB
and IBPB_ON_VMEXIT and simplifies the alternatives.

Fixes: fb3bd914 ("x86/srso: Add a Speculative RAS Overflow mitigation")
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/r/20230814121149.109557833@infradead.org