vhost_user: add VHOST_USER_PROTOCOL_F_INFLIGHT_SHMFD definition

We will enable vhost live recovery and packed ring feature in next
few days, however most of the code is in DPDK vhost library, so we
will not enable this feature for our internal vhost library, but
there are many users still use it, so we will not support the new
features with internal vhost library but will maintain it until
we can drop it in future, this is the first patch to do it, another
patch will be submitted until the packed ring patches being merged.

Change-Id: I50fa0314fd64fa5ee1f06a78e1495e2d0d50a0d0
Signed-off-by: Changpeng Liu <changpeng.liu@intel.com>
Reviewed-on: https://review.gerrithub.io/c/spdk/spdk/+/479502


Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>
Community-CI: SPDK CI Jenkins <sys_sgci@intel.com>
Reviewed-by: Jim Harris <james.r.harris@intel.com>
Reviewed-by: Tomasz Zawadzki <tomasz.zawadzki@intel.com>