trace: drop trailing empty strings

Also fix a typo in the virtio_balloon_handle_output() trace while here.

[The double-quoting was a limitation of the old tracetool.sh script.
The modern tracetool.py script does not require double-quotes at the end
of the line.  See commit cf85cf8e972f3ad79f203be4edb7968d6e052293
("trace: Format strings must begin/end with double quotes").
--Stefan]

Signed-off-by: Greg Kurz <gkurz@linux.vnet.ibm.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Message-id: 20160111173036.24764.59878.stgit@bahia.huguette.org
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
