net/virtio-user: add virtual device

net/virtio-user: add virtual device

Add a new virtual device named virtio-user, which can be used just like
eth_ring, eth_null, etc. To reuse the code of original virtio, we do
some adjustment in virtio_ethdev.c, such as remove key _static_ of
eth_virtio_dev_init() so that it can be reused in virtual device; and
we add some check to make sure it will not crash.

Configured parameters include:
  - queues (optional, 1 by default), number of queue pairs, multi-queue
    not supported for now.
  - cq (optional, 0 by default), not supported for now.
  - mac (optional), random value will be given if not specified.
  - queue_size (optional, 256 by default), size of virtqueues.
  - path (madatory), path of vhost user.

When enable CONFIG_RTE_VIRTIO_USER (enabled by default), the compiled
library can be used in both VM and container environment.

Examples:
path_vhost=<path_to_vhost_user> # use vhost-user as a backend

sudo ./examples/l2fwd/build/l2fwd -c 0x100000 -n 4 \
    --socket-mem 0,1024 --no-pci --file-prefix=l2fwd \
    --vdev=virtio-user0,mac=00:01:02:03:04:05,path=$path_vhost -- -p 0x1

Known issues:
 - Control queue and multi-queue are not supported yet.
 - Cannot work with --huge-unlink.
 - Cannot work with no-huge.
 - Cannot work when there are more than VHOST_MEMORY_MAX_NREGIONS(8)
   hugepages.
 - Root privilege is a must (mainly becase of sorting hugepages according
   to physical address).
 - Applications should not use file name like HUGEFILE_FMT ("%smap_%d").
 - Cannot work with vhost-net backend.

Signed-off-by: Huawei Xie <huawei.xie@intel.com>
Signed-off-by: Jianfeng Tan <jianfeng.tan@intel.com>
Acked-by: Neil Horman <nhorman@tuxdriver.com>
Acked-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>