tools/hv: Fix exit() error code

Linux native exit codes are 8-bit unsigned values.  exit(-1) results
in an exit code of 255, which is usually reserved for shells reporting
'command not found'.  Use the portable value EXIT_FAILURE.  (Not that
this matters much for a daemon.)

Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Cc: stable@vger.kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>