qemu: address todo comment in exec.c

exec.c has a comment 'XXX: optimize' for lduw_phys/stw_phys,
so let's do it, along the lines of stl_phys.

The reason to address 16 bit accesses specifically is that virtio relies
on these accesses to be done atomically, using memset as we do now
breaks this assumption, which is reported to cause qemu with kvm
to read wrong index values under stress.

https://bugzilla.redhat.com/show_bug.cgi?id=525323

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
