vhost_user_block: Move binary into vhost_user_block crate

The binary is still built in the same location but the source code and
the dependencies for it come from the vhost_user_block crate itself.

The binary will be built with:

`cargo build --all --bin vhost_user_block` or just `cargo build --all`

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

Cargo.lock

@@ -1430,7 +1430,7 @@ dependencies = [
 name = "vhost_user_block"
 version = "0.1.0"
 dependencies = [
- "bitflags 1.2.1",
+ "clap",
  "epoll",
  "libc",
  "log 0.4.8",

vhost_user_block/Cargo.toml

@@ -5,7 +5,7 @@ authors = ["The Cloud Hypervisor Authors"]
 edition = "2018"
 
 [dependencies]
-bitflags = "1.1.0"
+clap = { version = "2.33.1", features=["wrap_help"] }
 epoll = ">=4.0.1"
 libc = "0.2.71"
 log = "0.4.8"