x86/sev-es: Setup per-CPU GHCBs for the runtime handler

The runtime handler needs one GHCB per-CPU. Set them up and map them
unencrypted.

 [ bp: Touchups and simplification. ]

Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20200907131613.12703-42-joro@8bytes.org