KVM: x86: expose MOVDIR64B CPU feature into VM.

KVM: x86: expose MOVDIR64B CPU feature into VM.

MOVDIR64B moves 64-bytes as direct-store with 64-bytes write atomicity.
Direct store is implemented by using write combining (WC) for writing
data directly into memory without caching the data.

Availability of the MOVDIR64B instruction is indicated by the presence
of the CPUID feature flag MOVDIR64B (CPUID.0x07.0x0:ECX[bit 28]).

This patch exposes the movdir64b feature to the guest.

The release document ref below link:
https://software.intel.com/sites/default/files/managed/c5/15/\
architecture-instruction-set-extensions-programming-reference.pdf

Signed-off-by: Liu Jingqi <jingqi.liu@intel.com>
Cc: Xu Tao <tao3.xu@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>