virtio_blk: don't crash, report error if virtqueue is broken.

A bad implementation of virtio might cause us to mark the virtqueue
broken: we'll dev_err() in that case, and the device is useless, but
let's not BUG_ON().

ENOMEM or ENOSPC implies the ring is full, and we should try again
later (-ENOMEM is documented to happen, but doesn't, as we fall
through to ENOSPC).

EIO means it's broken.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>