vhost_user_block: Refactor vhost_user_block backend code into a new crate

Extract the majority of the code that provides the vhost-user-block
backend into its own crate and port the binary to use it.

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

vhost_user_block/Cargo.toml

@@ -0,0 +1,18 @@
+[package]
+name = "vhost_user_block"
+version = "0.1.0"
+authors = ["The Cloud Hypervisor Authors"]
+edition = "2018"
+
+[dependencies]
+bitflags = "1.1.0"
+epoll = ">=4.0.1"
+libc = "0.2.65"
+log = "0.4.8"
+qcow = { path = "../qcow" }
+vhost_user_backend = { path = "../vhost_user_backend" }
+vhost_rs = { path = "../vhost_rs" }
+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"