x86: emulator: long mode smsw tests

SMSW instruction on long-mode is performed according to the operand size, if
the destination operand is a register.  This patch tests whether it is
performed correctly instead of always using two-bytes operands (as KVM
previously did).  Note that when a dword destination operand is used, the
result is zero-extended to qword on long-mode.

Signed-off-by: Nadav Amit <namit@cs.technion.ac.il>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>