x86: realmode: fix lss test

Running lss with some random descriptor and then performing pop does not
work so well. Use mov instructions instead of push/pop pair.

Signed-off-by: Nadav Amit <namit@vmware.com>
Message-Id: <20200626092333.2830-4-namit@vmware.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>