replay: do not wake up loop too much
replay: do not wake up loop too much

Send wakeups only when a command is available.
This emulate better what a guest driver should do (append the command
to the ring and then signal).

Signed-off-by: Frediano Ziglio <55d48b080b2e443e395cde84d2c83b135a4ff48e@redhat.com>
Acked-by: Pavel Grunt <fbda40b445316123f12a5a6bd7556918ea74f4bc@redhat.com>

SPICE/spice,fgouget/spice,SPICE/spice,SPICE/spice,fgouget/spice,fgouget/spice,fgouget/spice