VZVirtualMachineInstance: Adjust silent success behavior of nested virt (#88)

Fixes #85

The virtualization bool on LinuxContainer mostly just forwards to
VZVirtualMachineInstance which today would silently take your boolean
and do nothing if the underlying platform doesn't have support for it.
This is (to me) arguably worse than erroring, as it gives the client a
false security that the setting is on, and they should have virt
capabilities in the container/guest now.

This change makes it so that we throw a ContainerizationError for this
case, with a code of .unsupported so it's checkable by a user if they
want more information on the "why".

Sources/Containerization/VZVirtualMachineInstance.swift

@@ -294,16 +294,18 @@ extension VZVirtualMachineInstance.Configuration {
             try mount.configure(config: &config)
         }
 
-        #if arch(arm64)
-
         let platform = VZGenericPlatformConfiguration()
-        if VZGenericPlatformConfiguration.isNestedVirtualizationSupported {
-            platform.isNestedVirtualizationEnabled = self.nestedVirtualization
+        // We shouldn't silently succeed if the user asked for virt and their hardware does
+        // not support it.
+        if !VZGenericPlatformConfiguration.isNestedVirtualizationSupported && self.nestedVirtualization {
+            throw ContainerizationError(
+                .unsupported,
+                message: "nested virtualization is not supported on the platform"
+            )
         }
+        platform.isNestedVirtualizationEnabled = self.nestedVirtualization
         config.platform = platform
 
-        #endif
-
         try config.validate()
         return config
     }