KVM: VMX: Check the posted interrupt capability

Detect the posted interrupt feature. If it exists, then set it in vmcs_config.

Signed-off-by: Yang Zhang <yang.z.zhang@Intel.com>
Reviewed-by: Gleb Natapov <gleb@redhat.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>