Expose Intel AVX-512 feature bits to guest. Also add checks for
xcr0 AVX512 related bits according to spec:
http://download-software.intel.com/sites/default/files/managed/71/2e/319433-017.pdfSigned-off-by: NChao Peng <chao.p.peng@linux.intel.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>