tools/virtio: add krealloc_array

krealloc_array is used in drivers/vhost/vringh.c, add it to avoid build
failure.

Drop WARN_ON_ONCE, because duplicated with the one in bug.h

Signed-off-by: Peng Fan <peng.fan@nxp.com>
Link: https://lore.kernel.org/r/20201209084205.24062-3-peng.fan@oss.nxp.com


Signed-off-by: Michael S. Tsirkin <mst@redhat.com>