QMP: Move STOP event into do_vm_stop()

I've introduced the STOP event in the main loop, this is wrong
as it will be only emitted if the io thread is enabled.

This fixes that by moving the STOP event to do_vm_stop().

Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
