cli: Fix default CPU argument

By default, and in order to avoid falling into the legacy CLI usage, the
CPU argument should at least include "boot=" to define the number of
CPUs.

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

src/main.rs

@@ -79,7 +79,7 @@ fn main() {
         }
     }
 
-    let default_vcpus = format! {"{}", config::DEFAULT_VCPUS};
+    let default_vcpus = format! {"boot={}", config::DEFAULT_VCPUS};
     let default_memory = format! {"size={}M", config::DEFAULT_MEMORY_MB};
     let default_rng = format! {"src={}", config::DEFAULT_RNG_SOURCE};