[PATCH 0 of 4] tools: build system fix indirect library usage and --as-needed

[Ian Campbell <ian.campbell@citrix.com>]

This series arranges for all libraries and binaries to only link
against those shared libraries which they use directly, relying on the
dynamic linker to do the right thing for indirect dependencies.

In order to do this I've pushed the tools/blktap2 UUID abstraction
down into libvhd and out of line to encapsulate all knowledge
regarding this abstraction into libvhd rather than expecting the users
of the library to do the right thing.

Having done this supporting --as-needed becomes a pretty trivial case
of ensuring links lines have the libraries in the correct order.

Tested by:
	* running xl, start/stop a guest
	* running xend, start/stop a guest
	* manually importing each python extension and calling
          whatever initialisation function I could find.