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>

Cargo.toml

@@ -42,6 +42,7 @@ mmio = ["vmm/mmio_support"]
 cmos = ["vmm/cmos"]
 fwdebug = ["vmm/fwdebug"]
 kvm = ["vmm/kvm"]
+io_uring = ["vmm/io_uring"]
 
 # Integration tests require a special environment to run in
 integration_tests = []