virtio_blk: Enable dump support. Partial sync with FreeBSD up to r252707.

virtio_blk: Enable dump support. Partial sync with FreeBSD up to r252707.

* Adds support for dumping to the virtio_blk driver.

* The dev.vtblk.X.writecache_mode sysctl can now be used to toggle write
  caching by the virtual-machine host.

* Add a workaround in sys/dev/virtual/virtio/virtio/virtqueue.c to avoid
  a serializer exit/enter pair while dumping.

* Reduce diff relative to FreeBSD's virtio_blk.c version.

* Adapt manpage, comment out description of the hw.vtblk.no_ident tunable.

* Updates are taken from FreeBSD's r252707, r252703 and r247829.