valgrind/s390x: avoid false positives on KVM_SET_FPU ioctl

struct kvm_fpu contains an alignment padding on s390x. Let's use a
designated initializer to avoid false positives from valgrind/memcheck.

Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

target-s390x/kvm.c

@@ -208,7 +208,7 @@ int kvm_arch_put_registers(CPUState *cs, int level)
     CPUS390XState *env = &cpu->env;
     struct kvm_sregs sregs;
     struct kvm_regs regs;
-    struct kvm_fpu fpu;
+    struct kvm_fpu fpu = {};
     int r;
     int i;