vmm: allow IGVM payload alongside a kernel

Previously, the payload validation rejected an IGVM file combined with
a kernel or firmware.

Relax this constraint to allow an IGVM carrying a firmware (e.g Oak
stage0) to be paired with a separate kernel image.

This enables fw_cfg-style boot where stage0 loads a kernel provided
through fw_cfg rather than embedded in the IGVM file itself.

Signed-off-by: Ruben Hakobyan <hruben@meta.com>

vmm/src/vm_config.rs

@@ -849,7 +849,7 @@ impl PayloadConfig {
         #[cfg(feature = "igvm")]
         {
             if self.igvm.is_some() {
-                if self.firmware.is_some() || self.kernel.is_some() {
+                if self.firmware.is_some() {
                     return Err(PayloadConfigError::IgvmPlusOtherPayloads);
                 }
                 return Ok(());