x86/smpboot: Allow parallel bringup for SEV-ES

x86/smpboot: Allow parallel bringup for SEV-ES

Enable parallel bringup for SEV-ES guests. The APs can't actually
execute the CPUID instruction directly during early startup, but they
can make the GHCB call directly instead, just as the VC trap handler
would do.

Factor out a prepare_parallel_bringup() function to help reduce the level
of complexity by allowing a simple 'return false' in the bail-out cases/

Thanks to Sabin for talking me through the way this works.

Suggested-by: Sabin Rapan <sabrapan@amazon.com>
Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>