vhost: fix build with kernel < 3.8

vhost: fix build with kernel < 3.8

Fix build error:
  virtio-net.c:80:89: error: ‘VIRTIO_NET_F_MQ’ undeclared here
  rte_virtio_net.h:109: error: ‘VIRTIO_NET_CTRL_MQ_VQ_PAIRS_MAX’ undeclared here

Above two virtio-net MQ macros are introduced since kernel v3.8.
For older kernel, we should not reference them directly, hence,
this patch introduced two wrapper macros, with proper values
being set depending on we support MQ or not.

Fixes: b09b198bfb5c ("vhost-user: announce queue number in message")

Reported-by: Yongjie Gu <yongjiex.gu@intel.com>
Signed-off-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>
Tested-by: David Marchand <david.marchand@6wind.com>