cpus.c: qemu_mutex_lock_iothread fix race condition at cpu thread init

When QEMU starts the RCU thread executes qemu_mutex_lock_thread
causing error "qemu:qemu_cpu_kick_thread: No such process" and exits.

This isn't occur frequently but in glibc the thread id can exist and
this not guarantee that the thread is on active/running state. If is
inserted a sleep(1) after newthread assignment [1] the issue appears.

So not make assumption that thread exist if first_cpu->thread is set
then change the validation of cpu to created that is set into cpu
threads (kvm, tcg, dummy).

[1] https://sourceware.org/git/?p=glibc.git;a=blob;f=nptl/pthread_create.c;h=d10f4ea8004e1d8f3a268b95cc0f8d93b8d89867;hb=HEAD#l621

Cc: qemu-stable@nongnu.org
Signed-off-by: Aníbal Limón <anibal.limon@linux.intel.com>
Message-Id: <1441313313-3040-1-git-send-email-anibal.limon@linux.intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
