wireshark: Adapt to tvb_new_subset() rename

wireshark: Adapt to tvb_new_subset() rename

In Wireshark commit of 7cd6906056922e4b8 (contained in v2.4.0)
the tvb_new_subset() function was renamed to
tvb_new_subset_length_caplen(). However, we can take the extra
step and rename to tvb_new_subset_remaining() directly (see
Wireshark commit 0ecfc7280cf3d7). The reasoning is that there is
no other protocol in the packet than libvirt. Therefore, from the
point that libvirt dissector takes over till the end of the
packet it's all libvirt packet.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Martin Kletzander <mkletzan@redhat.com>