valgrind: avoid false positives in KVM_GET_DIRTY_LOG ioctl

struct kvm_dirty_log contains padding fields that trigger false
positives in valgrind. 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>
