rcu: add rcu library

This includes a (mangled) copy of the urcu-qsbr code from liburcu.
The main changes are: 1) removing dependencies on many other header files
in liburcu; 2) removing for simplicity the tentative busy waiting in
synchronize_rcu, which has limited performance effects; 3) replacing
futexes in synchronize_rcu with QemuEvents for Win32 portability.
The API is the same as liburcu, so it should be possible in the future
to require liburcu on POSIX systems for example and use our copy only
on Windows.

Among the various versions available I chose urcu-qsbr, which has the
fastest rcu_read_{lock,unlock} but requires the program to manually
annotate quiescent points or intervals.  QEMU threads usually have easily
identified quiescent periods, so this should not be a problem.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
qemu-thread: register threads with RCU

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
rcu: add call_rcu

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
rcu: add rcutorture

A stress test program (works, too :)).

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
rcu: allow nested calls to rcu_thread_offline/rcu_thread_online

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
qemu-thread: report RCU quiescent states

Most threads will use mutexes and other sleeping synchronization primitives
(condition variables, semaphores, events) periodically.  For these threads,
the synchronization primitives are natural places to report a quiescent
state (possibly an extended one).

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
event loop: report RCU quiescent states

Threads that run event loops also have places that can sleep for an extended
time.  Place an extended quiescent state there.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
cpus: report RCU quiescent states

CPU threads have extended quiescent states while relinquishing control
to the accelerator (except TCG).

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
block: report RCU quiescent states

The aio workers may spend a long time executing I/O operations;
mark that time as an extended quiescent state.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
migration: report RCU quiescent states

The migration thread polls s->state periodically, it does not
use a mutex or condition variable, so it has to report quiescent
states manually.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>