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>
