tools: Refactor "xentoollog" into its own library

tools: Refactor "xentoollog" into its own library

In attempting to disaggregate libxenctrl I found that many of the
pieces were going to want access to this library, so split it out (as
it probably should always have been).

Various build adjustments are needed. In particular things which use
xtl_* themselves now need to explicity link against the library.

This has a nice side effect which is that users of libxl no longer
need to link against libxenctrl just to create a logger, which was
counter to the principal that applications using libxl shouldn't be
required to look behind the curtain. This means that xl no longer
links against libxenctrl.

The new library uses a version script to ensure that only expected
symbols are exported and to version them such that ABI guarantees can
be kept in the future.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>
[ ijc -- dropped QEMU_TRADITIONAL_REVISION update, this had since
         progressed to 569eac99e8dd which is after 9fad9ed28583, the
         commit needed here. ]