kvm: remove unnecessary assignments

Avoid these warnings from clang analyzer:
/src/qemu/target-i386/kvm.c:772:5: warning: Value stored to 'cwd' is never read
    cwd = swd = twd = 0;
/src/qemu/target-i386/kvm.c:772:11: warning: Although the value stored to 'swd' is used in the enclosing expression, the value is never actually read from 'swd'
    cwd = swd = twd = 0;
Signed-off-by: NBlue Swirl <blauwirbel@gmail.com>

target-i386/kvm.c

@@ -769,7 +769,7 @@ static int kvm_put_xsave(CPUState *env)
 
     xsave = qemu_memalign(4096, sizeof(struct kvm_xsave));
     memset(xsave, 0, sizeof(struct kvm_xsave));
-    cwd = swd = twd = 0;
+    twd = 0;
     swd = env->fpus & ~(7 << 11);
     swd |= (env->fpstt & 7) << 11;
     cwd = env->fpuc;