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)

(backported from commit bc226f07)
[smb: context and dropped guest_cpuid_has checks in svm.c]
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>