error: only prepend timestamp on stderr

The -msg timestamp=on option prepends a timestamp to error messages.
This is useful on stderr where it allows users to identify when an error
was raised.

Timestamps do not make sense on the monitor since error_report() is
called in response to a synchronous monitor command and the user already
knows "when" the command was issued.  Additionally, the rest of the
monitor conversation lacks timestamps so the error timestamp cannot be
correlated with other activity.

Only prepend timestamps on stderr.  This fixes libvirt's 'drive_del'
processing, which did not expect a timestamp.  Other QEMU monitor
clients are probably equally confused by timestamps on monitor error
messages.

Cc: Markus Armbruster <armbru@redhat.com>
Cc: Seiji Aguchi <seiji.aguchi@hds.com>
Cc: Frank Schreuder <fschreuder@transip.nl>
Cc: Daniel P. Berrange <berrange@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-Id: <1439212541-16997-1-git-send-email-stefanha@redhat.com>
Reviewed-by: Daniel P. Berrange <berrange@redhat.com>
Tested-by: Frank Schreuder <fschreuder@transip.nl>
Signed-off-by: Markus Armbruster <armbru@redhat.com>