qemu_cgroup: Drop ENOENT special case for RNG devices

qemu_cgroup: Drop ENOENT special case for RNG devices

When allowing or denying RNG device in CGroups there's a special
check if the backend device exists (errno == ENOENT) in which
case success is returned to caller. This is in contrast with the
rest of the functions and in fact wrong too - if the backend
device doesn't exist then QEMU will fail opening it. Might as
well signal error here.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Pavel Hrdina <phrdina@redhat.com>