build: Consolidate sev_snp feature usage

Since igvm is a required feature of sev_snp and also sev_snp is x86-64
only the cfg attributes at build time can be consolidated & simplified.

Signed-off-by: Rob Bradford <rbradford@meta.com>

hypervisor/src/mshv/mod.rs

@@ -447,10 +447,7 @@ pub struct MshvVcpu {
     #[cfg(target_arch = "x86_64")]
     msrs: Vec<MsrEntry>,
     vm_ops: Option<Arc<dyn vm::VmOps>>,
-    #[cfg_attr(
-        all(not(target_arch = "x86_64"), not(feature = "sev_snp")),
-        expect(dead_code)
-    )]
+    #[cfg_attr(not(target_arch = "x86_64"), expect(dead_code))]
     vm_fd: Arc<VmFd>,
     #[cfg(feature = "sev_snp")]
     ghcb: Option<Ghcb>,