linux-user: un-parent OBJECT(cpu) when closing thread

linux-user: un-parent OBJECT(cpu) when closing thread

While forcing the CPU to unrealize by hand does trigger the clean-up
code we never fully free resources because refcount never reaches
zero. This is because QOM automatically added objects without an
explicit parent to /unattached/, incrementing the refcount.

Instead of manually triggering unrealization just unparent the object
and let the device machinery deal with that for us.

Resolves: https://gitlab.com/qemu-project/qemu/-/issues/866
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Message-Id: <20220811151413.3350684-2-alex.bennee@linaro.org>