Connect stdout/stderr of Firecracker in Debug mode

[kzys]

Issue #, if available:

It would improve the situation described on #128, but it may not be enough to resolve the issue.

Description of changes:

The log is pretty noisy but may be useful for debugging.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[kzys]

The logs are like below.

time="2019-10-29T19:16:38.301605405Z" level=debug msg="2019-10-29T19:16:38.301479342 [anonymous-instance:DEBUG] vsock: epoll_handler::process_rx()" jailer=runc ociBundlePath=/run/firecracker-containerd/default/3 runcBinaryPath=/usr/local/bin/runc runtime=aws.firecracker vmID=3 vmm_stream=stdout
time="2019-10-29T19:16:38.301623830Z" level=debug msg="2019-10-29T19:16:38.301504397 [anonymous-instance:DEBUG] vsock muxer: RX pkt: [2, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 8, 0, 0, 64, 37, 42, 0, 0, 0, 0, 0, 0, 1, 0, 3, 0, 0, 0, 0, 0, 0, 0, 1, 0, 213, 28, 0, 0]" jailer=runc ociBundlePath=/run/firecracker-containerd/default/3 runcBinaryPath=/usr/local/bin/runc runtime=aws.firecracker vmID=3 vmm_stream=stdout
time="2019-10-29T19:16:38.301646248Z" level=debug msg="2019-10-29T19:16:38.301521841 [anonymous-instance:DEBUG] vsock: raising IRQ" jailer=runc ociBundlePath=/run/firecracker-containerd/default/3 runcBinaryPath=/usr/local/bin/runc runtime=aws.firecracker vmID=3 vmm_stream=stdout
time="2019-10-29T19:16:38.306646708Z" level=debug msg="[   77.519862] agent[735]: time=\"2019-10-29T19:16:38Z\" level=info msg=\"stopping ttrpc server\"" jailer=runc ociBundlePath=/run/firecracker-containerd/default/3 runcBinaryPath=/usr/local/bin/runc runtime=aws.firecracker vmID=3 vmm_stream=stdout
time="2019-10-29T19:16:38.309202566Z" level=debug msg="[   77.521980] agent[735]: time=\"2019-10-29T19:16:38Z\" level=error msg=\"failed to close ttrpc server\" error=\"context canceled\"" jailer=runc ociBundlePath=/run/firecracker-containerd/default/3 runcBinaryPath=/usr/local/bin/runc runtime=aws.firecracker vmID=3 vmm_stream=stdout
time="2019-10-29T19:16:38.311431696Z" level=debug msg="[   77.524770] agent[735]: time=\"2019-10-29T19:16:38Z\" level=info msg=\"shutting down agent\"" jailer=runc ociBundlePath=/run/firecracker-containerd/default/3 runcBinaryPath=/usr/local/bin/runc runtime=aws.firecracker vmID=3 vmm_stream=stdout

[sipsma]

LGTM

[mxpv]

LGTM