lib/vhost: rename rte_vhost_compat to rte_vhost_user

For some time already the DPDK rte_vhost interface was
accomodating for other types of devices than virtio-net.
rte_vhost_compat.c file contained the use of DPDK rte_vhost,
rather than workarounds. To make that clear it is now renamed
to rte_vhost_user.c.

This patch is first in series that reworks vhost library
with two goals in mind:

1) Refactor vhost and vhost-blk to no longer depend on rte_vhost.
All references to that API will be moved to rte_vhost_user.c.

2) Add a transport abstraction for virtio-blk devices.
vhost-blk will now be able to expose virtio-blk using multiple
implementations of the interface.
First one will be vhost_user that depends on DPDK rte_vhost library.

Signed-off-by: Tomasz Zawadzki <tomasz.zawadzki@intel.com>
Change-Id: Ib6d4e4a6352069fa76e6b017ec203dab75f887b8
Reviewed-on: https://review.spdk.io/gerrit/c/spdk/spdk/+/11052


Community-CI: Broadcom CI <spdk-ci.pdl@broadcom.com>
Community-CI: Mellanox Build Bot
Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>
Reviewed-by: Changpeng Liu <changpeng.liu@intel.com>
Reviewed-by: Jim Harris <james.r.harris@intel.com>
Reviewed-by: Konrad Sztyber <konrad.sztyber@intel.com>