virtio: rng: allow tasks to be killed that are waiting for rng input

Use wait_for_completion_killable() instead of wait_for_completion() when
waiting for the host to send us entropy.  Without this,

  # cat /dev/hwrng
  ^C

just hangs.

Signed-off-by: Amit Shah <amit.shah@redhat.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>