x86: emulator: Fix h_mem usage in tests_smsw

In emulator.c/tests_smsw, smsw (3) fails because h_mem isn't being set correctly
before smsw is called. By using the + constraint modifier for memory we can
ensure the compiler no longer optimizes out the assignment before smsw.

Signed-off-by: Chris J Arges <chris.j.arges@canonical.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>