xenconsoled: don't force newline with timestamps

xenconsoled: don't force newline with timestamps

Previously a newline was forced after every call to
write_with_timestamp. This could result in some ugly logs when a
single console line stretched over multiple calls. This change carries
some extra state to determine if we ended the last call to
write_with_timestamp on a newline, and prints the timestamp as
appropriate.

This also fixes a bug where logging hv messages was dependent on the
guest timestamp flag.

Signed-off-by: Cole Robinson <crobinso@redhat.com>