tools: Rationalise library soname versions.

tools: Rationalise library soname versions.

 * Arrange for the sonames of libxenstore, libxc, libfsimage and
   libblktap to be set from a single place in Config.mk.
 * Bumps the soname major version number to 3.2 in preparation for
   3.2 rc1 after which we do not expect to have ABI changes.

I have not done anything about libaio and libflask.  The former is
imported from elsewhere and if we're lucky will have sane versioning
upstream.

libflask has a very small interface and I'm hoping that the XSM
authors have been keeping some ABI discipline.

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