Cargo: switch vhost_rs to external crate

As cloud-hypervisor/vhost crate (dragonball branch) is ready to be used,
switch vhost_rs from internal crate to the external one.

Signed-off-by: Eryu Guan <eguan@linux.alibaba.com>

Cargo.toml

@@ -23,6 +23,7 @@ vm-device = { path = "vm-device" }
 vm-memory = { git = "https://github.com/rust-vmm/vm-memory" }
 vmm-sys-util = ">=0.3.1"
 vm-virtio = { path = "vm-virtio" }
+vhost_rs = { git = "https://github.com/cloud-hypervisor/vhost", branch = "dragonball", package = "vhost", features = ["vhost-user-slave"] }
 
 [dev-dependencies]
 ssh2 = "0.8.0"
@@ -32,10 +33,6 @@ tempdir= "0.3.7"
 lazy_static= "1.4.0"
 serde_json = ">=1.0.9"
 
-[dependencies.vhost_rs]
-path = "vhost_rs"
-features = ["vhost-user-slave"]
-
 [features]
 default = ["acpi", "pci", "cmos"]
 acpi = ["vmm/acpi"]
@@ -50,7 +47,6 @@ integration_tests = []
 members = [
     "arch",
     "devices",
-    "vhost_rs",
     "qcow",
     "pci",
     "vmm",