vhost: fix features definition location

vhost: fix features definition location

The vhost library provide an infrastructure in order to help the DPDK
users to manage vhost devices.

One of the infrastructure parts is the features enablement APIs.

Some features bits may be defined only in the internal file vhost.h in
case the kernel version doesn't include them.

Hence, user running on old kernel may not be able to manage thus
features.

Move all the feature bits definitions to the API file rte_vhost.h.

Fixes: db69be54b6ff ("vhost: hide internal code")
Fixes: 8d286dbeb8d7 ("vhost: fix multiple queue not enabled for old kernels")
Fixes: 3d3c6590b58c ("vhost: enable virtio MTU feature")
Fixes: 704098fc478c ("vhost: fix build with old kernels")
Cc: stable@dpdk.org
Signed-off-by: Matan Azrad <matan@mellanox.com>
Reviewed-by: Chenbo Xia <chenbo.xia@intel.com>
Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>