tools: Update sonames for 4.6 RCs

tools: Update sonames for 4.6 RCs

Update libxc to 4.6.
Update libxl to 4.6.
Update libxlu to 4.6.

I did
  git-grep 'MAJOR.*='
and also to check I had everything
  git-grep 'SONAME_LDFLAG' | egrep -v 'MAJOR' |less

The other, un-updated, libraries are:
  blktap2 (control, libvhd) 1.0  in-tree users only, no ABI changes
  libfsimage    1.0      no ABI changes
  libvchan      1.0      no ABI changes
  libxenstat    0.0 (!)  no ABI changes
  libxenstore   3.0      no ABI changes

My assertions "no ABI changes" are based on the output of
  git-diff origin/stable-4.5..staging .
and similar runes, sometimes limited to .h files.

Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>
CC: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>
---
v2: Bump libxlu too.  [ Reported by Wei Liu. ]