Allow backends to know features that can be used
Allow backends to know features that can be used

Previous to adding a a trait method to inform the backends of the acked
features backends can use features than the guest has not enabled which
could lead to unpredictable results.

Signed-off-by: Rob Bradford <c49e1dc14cd52e2644816db47123896e0ac79025@intel.com>

rust-vmm/vhost