cpus: run work items for all vCPUs if single-threaded

cpus: run work items for all vCPUs if single-threaded

This avoids the following I/O thread deadlock:

1) the I/O thread calls run_on_cpu for CPU 3 from a timer.  single_tcg_halt_cond
is signaled

2) CPU 1 is running and exits.  It finds no work item and enters CPU 2

3) because the I/O thread is stuck in run_on_cpu, the round-robin kick
timer never triggers, and CPU 3 never runs the work item

4) run_on_cpu never completes

Reviewed-by: Emilio G. Cota <cota@braap.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>