KVM: SVM: Implement VIRT_SPEC_CTRL support for SSBD

KVM: SVM: Implement VIRT_SPEC_CTRL support for SSBD

Expose the new virtualized architectural mechanism, VIRT_SSBD, for using
speculative store bypass disable (SSBD) under SVM.  This will allow guests
to use SSBD on hardware that uses non-architectural mechanisms for enabling
SSBD.

[ tglx: Folded the migration fixup from Paolo Bonzini ]

Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
CVE-2018-3639 (x86)

(cherry-picked from commit bc226f07dcd3c9ef0b7f6236fe356ea4a9cb4769)
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>