spice: avoid spice runtime assert

spice: avoid spice runtime assert

The Spice server doesn't like to be started or stopped twice . It
aborts with:

(process:6191): Spice-ERROR **: 19:29:35.912: red-worker.c:623:handle_dev_start: assertion `!worker->running' failed

It's easy to avoid that situation since qemu spice_display_is_running
tracks the server state.

After the commit "spice: do not stop spice if VM is paused", it will
be possible to pause and resume the VM, and this will call
qemu_spice_display_start() twice. The easiest is to add a check for
spice_display_is_running with this patch to avoid the assert.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Tested-by: Victor Toso <victortoso@redhat.com>
Message-id: 20190221110703.5775-4-marcandre.lureau@redhat.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>