Tools: hv: fix snprintf warning in kvp_daemon

Increase buffer size so that "_{-INT_MAX}" will fit.
Spotted by the gcc7 snprintf checker.

Signed-off-by: Olaf Hering <olaf@aepfle.de>
Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>