perf scripts python: exported-sql-viewer.py: Add LookupModel()

Add LookupModel() to find a model in the model cache without creating it.

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Link: http://lore.kernel.org/lkml/20190821083216.1340-2-adrian.hunter@intel.com


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