linux-user: properly "unrealize" vCPU object

linux-user: properly "unrealize" vCPU object

We shouldn't be messing around with the CPU list in linux-user save
for the very special case of do_fork(). When threads end we need to
properly follow QOM object lifetime handling and allow the eventual
cpu_common_unrealizefn to both remove the CPU and ensure any clean-up
actions are taken place, for example calling plugin exit hooks.

There is still a race condition to avoid so use the linux-user
specific clone_lock instead of the cpu_list_lock to avoid it.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Cc: Nikolay Igotti <igotti@gmail.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Daniel P. Berrange <berrange@redhat.com>
Cc: Eduardo Habkost <ehabkost@redhat.com>
Cc: Markus Armbruster <armbru@redhat.com>
Message-Id: <20200520140541.30256-14-alex.bennee@linaro.org>