spice-option: warn on command line failures of usbredir

As spice_usb_device_manager_get() can fail for different reasons, we
should ignore and continue but at least give a warning.

Signed-off-by: Victor Toso <victortoso@redhat.com>
Acked-by: Frediano Ziglio <fziglio@redhat.com>
2 jobs for master in 3 minutes and 13 seconds (queued for 50 seconds)
Status Job ID Name Coverage
  Test
passed #446808
fedora

00:03:13

passed #446809
windows

00:02:05