target/i386/kvm: Silence warning from Valgrind about uninitialized bytes

target/i386/kvm: Silence warning from Valgrind about uninitialized bytes

When I run QEMU with KVM under Valgrind, I currently get this warning:

 Syscall param ioctl(generic) points to uninitialised byte(s)
    at 0x95BA45B: ioctl (in /usr/lib64/libc-2.28.so)
    by 0x429DC3: kvm_ioctl (kvm-all.c:2365)
    by 0x51B249: kvm_arch_get_supported_msr_feature (kvm.c:469)
    by 0x4C2A49: x86_cpu_get_supported_feature_word (cpu.c:3765)
    by 0x4C4116: x86_cpu_expand_features (cpu.c:5065)
    by 0x4C7F8D: x86_cpu_realizefn (cpu.c:5242)
    by 0x5961F3: device_set_realized (qdev.c:835)
    by 0x7038F6: property_set_bool (object.c:2080)
    by 0x707EFE: object_property_set_qobject (qom-qobject.c:26)
    by 0x705814: object_property_set_bool (object.c:1338)
    by 0x498435: pc_new_cpu (pc.c:1549)
    by 0x49C67D: pc_cpus_init (pc.c:1681)
  Address 0x1ffeffee74 is on thread 1's stack
  in frame #2, created by kvm_arch_get_supported_msr_feature (kvm.c:445)

It's harmless, but a little bit annoying, so silence it by properly
initializing the whole structure with zeroes.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>