Rename package to vhost-user-backend

Rename package from vhost_user_backend to vhost-user-backend to
accommodate to rust-vmm's conventions.

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

Author: slp

Cargo.toml

@@ -1,5 +1,5 @@
 [package]
-name = "vhost_user_backend"
+name = "vhost-user-backend"
 version = "0.1.0"
 authors = ["The Cloud Hypervisor Authors"]
 edition = "2018"