Convert error_report*_err() to warn_report*_err()

Convert error_report*_err() to warn_report*_err()

Convert all uses of error_report*_err("Warning:"... to use
warn_report*_err() instead. This helps standardise on a single
method of printing warnings to the user.

Signed-off-by: Alistair Francis <[email protected]>
Cc: Paolo Bonzini <[email protected]>
Cc: Richard Henderson <[email protected]>
Cc: Eduardo Habkost <[email protected]>
Cc: "Michael S. Tsirkin" <[email protected]>
Reviewed-by: Philippe Mathieu-Daudé <[email protected]>
Reviewed-by: Eduardo Habkost <[email protected]>
Reviewed-by: Markus Armbruster <[email protected]>
Message-Id: <d8e088757186955f40f04ec4f4be7f640d3c8660.1499866456[email protected]>
Signed-off-by: Markus Armbruster <[email protected]>