tools/hv: Fix string types

Initial patch by Ben Hutchings <ben@decadent.org.uk>

Standard C strings are arrays of char, not __u8 (unsigned char).
Declare variables and parameters accordingly, and add the necessary
casts.

Signed-off-by: Tomas Hozza <thozza@redhat.com>
Acked-by: K. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>