vmm: Flag --disk should be optional

Now that cloud-hypervisor VMM supports virtio-pmem, it can directly
boot a VM from an image exposed as a persistent memory block device.

That's why there is no need to force the --disk option as being
mandatory.

Fixes #90

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

src/main.rs

@@ -100,10 +100,7 @@ fn main() {
 
     let cmdline = cmd_arguments.value_of("cmdline");
 
-    let disks: Vec<&str> = cmd_arguments
-        .values_of("disk")
-        .expect("Missing argument: disk. Provide at least one")
-        .collect();
+    let disks: Option<Vec<&str>> = cmd_arguments.values_of("disk").map(|x| x.collect());
 
     let net: Option<Vec<&str>> = cmd_arguments.values_of("net").map(|x| x.collect());