libvirt probing fails due to assertion failure with KVM and 'none' machine type

Using libvirt on Ubuntu 19.10, I get the following error when I try to set <emulator> to the latest qemu from git (commit dc65a5bdc9):

    error: internal error: Failed to start QEMU binary /usr/local/bin/qemu-system-x86_64 for probing: /home/joey/git/qemu/target/i386/kvm.c:2176:kvm_arch_init: Object 0x564bfd5c3200 is not an instance of type x86-machine

Qemu command line to reproduce:

    sudo x86_64-softmmu/qemu-system-x86_64 -machine 'none,accel=kvm'

Commit ed9e923c3c (Dec 12, 2019) introduced the issue by removing an object_dynamic_cast call.  In this scenario, kvm_arch_init is passed an instance of "none-machine" instead of "x86-machine".

The following one-line change to target/i386/kvm.c reintroduces the cast:

     if (kvm_check_extension(s, KVM_CAP_X86_SMM) &&
+        object_dynamic_cast(OBJECT(ms), TYPE_X86_MACHINE) &&
         x86_machine_is_smm_enabled(X86_MACHINE(ms))) {
         smram_machine_done.notify = register_smram_listener;
         qemu_add_machine_init_done_notifier(&smram_machine_done);
     }