io_uring: Add new feature gate

By adding a new io_uring feature gate, we let the user the possibility
to choose if he wants to enable the io_uring improvements or not.
Since the io_uring feature depends on the availability on recent host
kernels, it's better if we leave it off for now.

As soon as our CI will have support for a kernel 5.6 with all the
features needed from io_uring, we'll enable this feature gate
permanently.

Signed-off-by: Sebastien Boeuf <sebastien.boeuf@intel.com>

block_util/Cargo.toml

@@ -4,6 +4,10 @@ version = "0.1.0"
 authors = ["The Cloud Hypervisor Authors"]
 edition = "2018"
 
+[features]
+default = []
+io_uring = []
+
 [dependencies]
 io-uring = { git = "https://github.com/tokio-rs/io-uring.git", branch = "0.4" }
 libc = "0.2.74"