use qemu_cpu_kick instead of cpu_exit or qemu_cpu_kick_thread

use qemu_cpu_kick instead of cpu_exit or qemu_cpu_kick_thread

Use the same API to trigger interruption of a CPU, no matter if
under TCG or KVM.  There is no difference: these calls come from
the CPU thread, so the qemu_cpu_kick calls will send a signal
to the running thread and it will be processed synchronously,
just like a call to cpu_exit.  The only difference is in the
overhead, but neither call to cpu_exit (now qemu_cpu_kick)
is in a hot path.

Reviewed-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>