Add QA tests for gnt-instance modify

This patch adds QA tests for most of the possible parameters in the
instance modify operation (exception being the MAC), and modifies the
sample QA file to run this test.

It also tests the no-modification test, but that is a weak one: we only
test that the exit code is one, not that the command gave a proper
response ("... please give at least one parameter") as opposed to a
traceback.

Reviewed-by: imsnah