KVM: selftests: Avoid flooding debug log while populating memory

Peter Xu pointed out that a log message printed while waiting for the
memory population phase of the dirty_log_perf_test will flood the debug
logs as there is no delay after printing the message. Since the message
does not provide much value anyway, remove it.

Reviewed-by: Jacob Xu <jacobhxu@google.com>

Signed-off-by: Ben Gardon <bgardon@google.com>
Message-Id: <20210112214253.463999-3-bgardon@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>