qemu_vm: wait until VM is paused in case -no-shutdown is being used

This should help us in the main use case of the kill_vm_only_when_paused
code: to make the shutdown code wait until the VM is paused instead of
waiting until the VM is killed, when -no-shutdown is used.
Signed-off-by: NEduardo Habkost <ehabkost@redhat.com>