bump libxenctrl and libxenguest versions

bump libxenctrl and libxenguest versions

In c/s 19009 an incompatible change was made to the ABI of libelf-*,
which are included in libxenctrl and libxenguest.  To avoid client
programs (such as python/lowlevel/xc/xc.so) picking up a wrong
version and perhaps crashing, the soname should be changed.

I have chosen the version number 3.4 as xen-unstable is currently the
pre-3.4 branch.  I assume we don't care about API/ABI stability within
an unstable branch.

Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>