virtio: Fix comment typo 'CONFIG_S_FAILED'

Without the VIRTIO_ prefix CONFIG_S_FAILED looks like a Kconfig macro.
So use that prefix here too.

Signed-off-by: Paul Bolle <pebolle@tiscali.nl>
Acked-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>