libvirt_vm: Fix VM destroy function to deal with 'crashed' VM

If VM state is 'crashed', the qemu process still exists, and start
VM will fail. So move virsh.destroy() function out of 'if' scope to
destroy VM directly, as 'ignore_status=True' by default, so it will
not raise error if domain is already shutoff.

Signed-off-by: Yanbing Du <ydu@redhat.com>