cpus: be more paranoid in avoiding deadlocks

For good measure, ensure that the following sequence:

   thread 1 calls qemu_mutex_lock_iothread
   thread 2 calls qemu_mutex_lock_iothread
   VCPU thread are created
   VCPU thread enters execution loop

results in the VCPU threads letting the other two threads run
and obeying iothread_requesting_mutex even if the VCPUs are
not halted.  To do this, check iothread_requesting_mutex
before execution starts.

Tested-by: Leon Alrae <leon.alrae@imgtec.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>