iotests: Fix 051 output after qdev_init_nofail() removal

iotests: Fix 051 output after qdev_init_nofail() removal

Commit 96927c744 replaced qdev_init_nofail() call by
isa_realize_and_unref() which has a different error
message. Update the test output accordingly.

Gitlab CI error after merging b77b5b3dc7:
https://gitlab.com/qemu-project/qemu/-/jobs/597414772#L4375

Reported-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20200616154949.6586-1-philmd@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>