x86/hvm: add pkeys support for cpuid handling

x86/hvm: add pkeys support for cpuid handling

This patch adds pkeys support for cpuid handing.

Pkeys hardware support is CPUID.7.0.ECX[3]:PKU. software support is
CPUID.7.0.ECX[4]:OSPKE and it reflects the support setting of CR4.PKE.

X86_FEATURE_OSXSAVE depends on guest X86_FEATURE_XSAVE, but cpu_has_xsave
function reflects hypervisor X86_FEATURE_XSAVE, it is fixed too.

Signed-off-by: Huaitong Han <huaitong.han@intel.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>