perf scripts python: exported-sql-viewer.py: Factor out CallGraphModelBase

Factor out a base class CallGraphModelBase from CallGraphModel, so that
CallGraphModelBase can be reused.

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Link: https://lkml.kernel.org/n/tip-76eybebzjwvgnadkm2oufrqi@git.kernel.org


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