i386: Add PKU on Skylake-Server CPU model

i386: Add PKU on Skylake-Server CPU model

As the release document ref below link (page 13):
https://software.intel.com/sites/default/files/managed/c5/15/\
architecture-instruction-set-extensions-programming-reference.pdf

PKU is supported in Skylake Server (Only Server) and later, and
on Intel(R) Xeon(R) Processor Scalable Family. So PKU is supposed
to be in Skylake-Server CPU model. And PKU's CPUID has been
exposed to QEMU. But PKU can't be find in Skylake-Server CPU
model in the code. So this patch will fix this issue in
Skylake-Server CPU model.

Signed-off-by: Tao Xu <tao3.xu@intel.com>
Message-Id: <5014b57f834dcfa8fd3781504d98dcf063d54fde.1540801392.git.tao3.xu@intel.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>