Revert 16560:822d4ec5cfb1.

Revert 16560:822d4ec5cfb1.

Preference seems to be to update library versions only when the ABI
changes. Only libxenctrl/libxenguest have their version number changed
to 3.2.0, as a reminder that they change on pretty much every major
release.

libfsimage/libxenstore/libblktap have not changed substantially
recently, hence the ABI has not changed, and version number stays the
same as in Xen 3.1 (and before).

Signed-off-by: Keir Fraser <keir.fraser@citrix.com>