When all cpus are sleeping (e.g in WFI), to avoid a deadlock
in the main_loop, wake it up in order to start the warp timer.

Signed-off-by: Clement Deschamps <clement.deschamps@greensocs.com>
Message-Id: <20181021142103.19014-1-clement.deschamps@greensocs.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>