dump: Assert cpu_get_note_size() can't fail

dump: Assert cpu_get_note_size() can't fail

The only way cpu_get_note_size() can return a negative value is
integer overflow in the non-stub versions, which is a programming
error.  The stub version is not actually reachable, because the
cpu_get_dump_info() stub will fail first.  Use assert().  This gets
rid of another use of QERR_UNSUPPORTED.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20230207075115.1525-4-armbru@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Juan Quintela <quintela@redhat.com>