x86: Intel AVX512_BF16 feature enabling

x86: Intel AVX512_BF16 feature enabling

Intel CooperLake cpu adds AVX512_BF16 instruction, defining as
CPUID.(EAX=7,ECX=1):EAX[bit 05].

The patch adds a property for setting the subleaf of CPUID leaf 7 in
case that people would like to specify it.

The release spec link as follows,
https://software.intel.com/sites/default/files/managed/c5/15/\
architecture-instruction-set-extensions-programming-reference.pdf

Signed-off-by: Jing Liu <jing2.liu@linux.intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>