Tools: hv: remove unused bytes_written from kvp_update_file()

fwrite() does not actually return the number of bytes written and
this value is being ignored anyway and ferror() is being called to
check for an error. As we assign to this variable and never use it
we get the following compile-time warning:
hv_kvp_daemon.c:149:9: warning: variable .bytes_written. set but not used [-Wunused-but-set-variable]
Remove bytes_written completely.

Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>