wireshark: Switch to tvb_bytes_to_str()

wireshark: Switch to tvb_bytes_to_str()

When the dissector sees a byte sequence that is either an opaque
data (xdr_opaque) or a byte sequence (xdr_bytes) it formats the
bytes as a hex numbers using our own implementation. But
wireshark already provides a function for it: tvb_bytes_to_str().
NB, the reason why it returns a const string is so that callers
don't try to free it - the string is allocated using an allocator
which will decide when to free it.

The wireshark formatter was introduced in wireshark commit of
v1.99.2~479 and thus is present in the version we require at
least (2.6.0).

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Jiri Denemark <jdenemar@redhat.com>