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 <alistair.francis@xilinx.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Richard Henderson <rth@twiddle.net>
Cc: Eduardo Habkost <ehabkost@redhat.com>
Cc: "Michael S. Tsirkin" <mst@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <d8e088757186955f40f04ec4f4be7f640d3c8660.1499866456.git.alistair.francis@xilinx.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>