perf scripts python: exported-sql-viewer.py: Improve TreeModel abstraction

Instead of passing the tree root, get it from a method that can be
implemented in any derived class.

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Link: https://lkml.kernel.org/n/tip-ovcv28bg4mt9swk36ypdyz14@git.kernel.org


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>