KVM: VMX: use correct vmcs_read/write for guest segment selector/base

Guest segment selector is 16 bit field and guest segment base is natural
width field. Fix two incorrect invocations accordingly.

Without this patch, build fails when aggressive inlining is used with ICC.

Cc: stable@vger.kernel.org
Signed-off-by: Chao Peng <chao.p.peng@linux.intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>