KVM: selftests: Always clear dirty bitmap after iteration

We used not to clear the dirty bitmap before because KVM_GET_DIRTY_LOG
would overwrite it the next time it copies the dirty log onto it.
In the upcoming dirty ring tests we'll start to fetch dirty pages from
a ring buffer, so no one is going to clear the dirty bitmap for us.

Reviewed-by: Andrew Jones <drjones@redhat.com>
Signed-off-by: Peter Xu <peterx@redhat.com>
Message-Id: <20201001012228.5916-1-peterx@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>