qapi: Improve input_type_enum()'s error message

qapi: Improve input_type_enum()'s error message

The error message claims the parameter is invalid:

    $ qemu-system-x86_64 -object qom-type=nonexistent
    qemu-system-x86_64: -object qom-type=nonexistent: Invalid parameter 'nonexistent'

What's wrong is actually the *value* 'nonexistent'.  Improve the
message to

    qemu-system-x86_64: -object qom-type=nonexistent: Parameter 'qom-type' does not accept value 'nonexistent'

Fixes: https://gitlab.com/qemu-project/qemu/-/issues/608
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20211020180231.434071-1-armbru@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>