vhost_user_net: don't claim to impl all features

Even if the vhost-user-net device did implement all currently-defined
features, it would be very short-sighted to use ::all(), because if a
new feature was defined later, the device would start claiming to
implement it even though it didn't.

More practically, claiming to implement all features breaks using QEMU
with the cloud-hypervisor vhost-user-net backend, because QEMU will
negotiate VHOST_USER_PROTOCOL_F_SLAVE_REQ, and then break when the
communication channel isn't actually set up.

I wasn't sure exactly which features the backend should claim to
implement, though.  Definitely MQ, and I'm fairly certain none of the
features I've ommitted are implemented.  But I'm not sure about
REPLY_ACK.  As far as I can tell it should be implemented entirely by
the vhost crate, with no cooperation required from the vhost-user-net
backend itself, so there should be no reason to let a frontend use it
if it wants to.  But despite this, neither vhost-user-fs nor
vhost-user-blk claims to implement it.

Signed-off-by: Alyssa Ross <hi@alyssa.is>

vhost_user_net/src/lib.rs

@@ -185,7 +185,7 @@ impl VhostUserBackend for VhostUserNetBackend {
     }
 
     fn protocol_features(&self) -> VhostUserProtocolFeatures {
-        VhostUserProtocolFeatures::all()
+        VhostUserProtocolFeatures::MQ | VhostUserProtocolFeatures::REPLY_ACK
     }
 
     fn set_event_idx(&mut self, _enabled: bool) {}