configure: disable internal rte_vhost on FreeBSD

On master DPDK (d03d8622d), building SPDK on FreeBSD
fails due to attempt to build internal rte_vhost.

This started occuring with SPDK patch:
28099e0e "make: add dependencies for the rte_vhost build."

This patch disables VHOST_INTERNAL_LIB on FreeBSD,
along with vhost/virtio.

Errors seen:
./configure --enable-debug --enable-werror --with-fio=/usr/src/fio --without-isal --with-dpdk=/var/jenkins/workspace/Other_systems/freebsd_autotest/dpdk/build
Using default SPDK env in /var/jenkins/workspace/Other_systems/freebsd_autotest/spdk/lib/env_dpdk
Notice: DPDK's rte_vhost not found or version < 19.05, using internal, legacy rte_vhost library.
Vhost is only supported on Linux. Disabling it.
Virtio is only supported on Linux. Disabling it.
...
gmake -j4
...
  CC lib/rte_vhost/socket.o
In file included from socket.c:52:
./vhost.h:41:10: fatal error: 'linux/vhost.h' file not found
#include <linux/vhost.h>
         ^~~~~~~~~~~~~~~
....
  CC lib/rte_vhost/vhost_user.o
vhost_user.c:41:10: fatal error: 'asm/mman.h' file not found
#include <asm/mman.h>
         ^~~~~~~~~~~~

Change-Id: Ia2f5fd98a82412a5691bc0f2201f7259a45d2b4d
Signed-off-by: Tomasz Zawadzki <tomasz.zawadzki@intel.com>
Reviewed-on: https://review.gerrithub.io/c/spdk/spdk/+/468043


Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>
Reviewed-by: Seth Howell <seth.howell@intel.com>
Reviewed-by: Jim Harris <james.r.harris@intel.com>
Reviewed-by: Ben Walker <benjamin.walker@intel.com>