x86/srso: Don't probe microcode in a guest

x86/srso: Don't probe microcode in a guest

To support live migration, the hypervisor sets the "lowest common
denominator" of features.  Probing the microcode isn't allowed because
any detected features might go away after a migration.

As Andy Cooper states:

  "Linux must not probe microcode when virtualised.  What it may see
  instantaneously on boot (owing to MSR_PRED_CMD being fully passed
  through) is not accurate for the lifetime of the VM."

Rely on the hypervisor to set the needed IBPB_BRTYPE and SBPB bits.

Fixes: 1b5277c0ea0b ("x86/srso: Add SRSO_NO support")
Suggested-by: Andrew Cooper <andrew.cooper3@citrix.com>
Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/r/3938a7209606c045a3f50305d201d840e8c834c7.1693889988.git.jpoimboe@kernel.org