vhost_user_backend: Add helpers for EVENT_IDX

Add helpers to Vring and VhostUserSlaveReqHandler for EVENT_IDX, so
consumers of this crate can make use of this feature.

Signed-off-by: Sergio Lopez <slp@redhat.com>

vhost_user_backend/Cargo.toml

@@ -13,6 +13,7 @@ mmio_support = ["vm-virtio/mmio_support"]
 epoll = ">=4.0.1"
 libc = "0.2.66"
 log = "0.4.8"
+virtio-bindings = "0.1.0"
 vm-memory = { git = "https://github.com/rust-vmm/vm-memory" }
 vm-virtio = { path = "../vm-virtio" }
 vmm-sys-util = ">=0.3.1"
@@ -20,4 +21,3 @@ vmm-sys-util = ">=0.3.1"
 [dependencies.vhost_rs]
 path = "../vhost_rs"
 features = ["vhost-user-slave"]
-