virtio-devices, vmm: Fix the '--seccomp false' option

We are relying on applying empty 'seccomp' filters to support the
'--seccomp false' option, which will be treated as an error with the
updated 'seccompiler' crate. This patch fixes this issue by explicitly
checking whether the 'seccomp' filter is empty before applying the
filter.

Signed-off-by: Bo Chen <chen.bo@intel.com>

vmm/src/lib.rs

@@ -263,7 +263,9 @@ pub fn start_vmm_thread(
         .name("vmm".to_string())
         .spawn(move || {
             // Apply seccomp filter for VMM thread.
-            apply_filter(&vmm_seccomp_filter).map_err(Error::ApplySeccompFilter)?;
+            if !vmm_seccomp_filter.is_empty() {
+                apply_filter(&vmm_seccomp_filter).map_err(Error::ApplySeccompFilter)?;
+            }
 
             let mut vmm = Vmm::new(
                 vmm_version.to_string(),