Fix the lexical priority of operators

As python dictionary has no defined key ordering the
GenericEvaluator.get_operators could return the operators in
bad order. (for example "<" before "<<")

This caused << to be parsed as two < elements and resulted
in parse errors.

This patch fixes it by ordering the returned value by string
size - longer names first.

Change-Id: I7dcb7b110b994b351534ccca4da6dc4b79e20895
Signed-off-by: Martin Sivak <msivak@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/16097
Reviewed-by: Adam Litke <agl@us.ibm.com>