qemu.py: avoid writing to stdout/stderr

This module should not write directly to stdout/stderr. Instead, it
should either raise exceptions or just log the messages and let the
callers handle them and decide what to do. For example, scripts could
choose to send the log messages stderr or/and write them to a file if
verbose or debugging mode is enabled.

This patch replaces the writes to stderr by an exception in the
send_fd_scm() when _socket_scm_helper is not set or not present. In the
same method, the subprocess Popen will now redirect the stdout/stderr to
logging.debug instead of writing to system stderr. As consequence, since
the Popen.communicate() is now used (in order to get the stdout), the
further call to wait() became redundant and was replaced by
Popen.returncode.

The shutdown() message on negative exit code will now be logged
to logging.warn instead of written to system stderr.

Signed-off-by: Amador Pahim <apahim@redhat.com>
Message-Id: <20170901112829.2571-3-apahim@redhat.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
