KVM: x86 emulator: disable writeback for TEST

The TEST instruction doesn't write its destination operand.  This
could cause problems if an MMIO register was accessed using the TEST
instruction.  Recently Windows XP was observed to use TEST against
the APIC ICR; this can cause spurious IPIs.

Signed-off-by: Avi Kivity <avi@redhat.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>