x86/CPU/AMD: Add the Secure Encrypted Virtualization CPU feature
Update the CPU features to include identifying and reporting on the
Secure Encrypted Virtualization (SEV) feature. SEV is identified by
CPUID 0x8000001f, but requires BIOS support to enable it (set bit 23 of
MSR_K8_SYSCFG and set bit 0 of MSR_K7_HWCR). Only show the SEV feature
as available if reported by CPUID and enabled by BIOS.