Just after boot a focal image panics propolis

Running latest Omicron (on sn21) I encountered a panic in propolis-server after a first boot of a newly created Ubuntu Focal instance.

The tail of the propolis-server log looks like this:

Dec 28 03:06:02.914 INFO piix3pm ignored cfg read, register: SmbusBase, offset: 0, dev: chipset
Dec 28 03:06:02.914 INFO piix3pm ignored cfg read, register: DevResA, offset: 0, dev: chipset
Dec 28 03:06:02.915 INFO piix3pm ignored cfg read, register: DevResB, offset: 0, dev: chipset
Dec 28 03:06:02.915 INFO piix3pm ignored cfg read, register: DevResC, offset: 0, dev: chipset
Dec 28 03:06:02.915 INFO piix3pm ignored cfg read, register: DevResI, offset: 0, dev: chipset
Dec 28 03:06:02.915 INFO piix3pm ignored cfg read, register: DevResJ, offset: 0, dev: chipset
Dec 28 03:06:03.125 INFO rdmsr, msr: 3221225705, vcpu: 0, component: vcpu_tasks
Dec 28 03:06:43.620 WARN [1] flow control start 
Dec 28 03:06:44.621 WARN [1] flow control sending more work
Dec 28 03:06:44.621 WARN [1] flow control end 
thread 'vcpu-2' panicked at 'vCPU 2: Unhandled VM exit: InstEmul(InstEmul { inst_data: [0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0], len: 0 })', bin/propolis-server/src/lib/vm/mod.rs:450:9
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
[ Dec 27 19:06:46 Stopping because all processes in service exited. ]
[ Dec 27 19:06:46 Executing stop method (:kill). ]

The stack of the thread that panic'd is:

-------------- thread# 140 / lwp# 140 [vcpu-2] ---------------                              
 fffff5ffef15863a _lwp_kill () + a                                                          
 fffff5ffef0ec58e raise (6) + 1e                                                            
 fffff5ffef0c5ff8 abort () + 58                                                             
 0000000001d2e7c9 panic_abort::__rust_start_panic::abort::hc1d5f3609ef6507d () + 9          
 0000000001d2e7b9 __rust_start_panic () + 9                                                 
 0000000001d19fac rust_panic () + 1c                                                        
 0000000001d19e34 std::panicking::rust_panic_with_hook::h3e4550c354b0930e () + 2c4          
 0000000001d19b56 std::panicking::begin_panic_handler::{{closure}}::he4c4fc5cc7d7a749 () + c6                                                                                           
 0000000001d16ef0 std::sys_common::backtrace::__rust_end_short_backtrace::hf95a242dcdd81a73 () + 20                                                                                     
 0000000001d19871 rust_begin_unwind () + 71                                                 
 0000000001d71a83 core::panicking::panic_fmt::h8e01a776a1dd4222 () + 33                     
 0000000000f12cf8 propolis_server::vm::SharedVmState::unhandled_vm_exit::hc565154474be8e59 () + 68                                                                                      
 00000000010532ea propolis_server::vcpu_tasks::VcpuTasks::vcpu_loop::h3e160f3429fdb39d () + 9ba                                                                                         
 000000000102a9b7 std::sys_common::backtrace::__rust_begin_short_backtrace::h00a3785c156ed8ed () + 57                                                                                   
 0000000000f99f8d core::ops::function::FnOnce::call_once{{vtable.shim}}::h1d68dceaad48552a () + bd                                                                                      
 0000000001d22d97 std::sys::unix::thread::Thread::new::thread_start::he5f444ef683dcd93 () + 27                                                                                          
 fffff5ffef15121c _thrp_setup (fffff5ffee734a40) + 6c
 fffff5ffef151530 _lwp_start ()

Reproduce the first boot of a newly created Ubuntu Focal instance and inspect the panic at bin/propolis-server/src/lib/vm/mod.rs:450, using the vcpu_tasks::VcpuTasks::vcpu_loop stack as context. Trace the unhandled InstEmul VM exit and verify that the instance completes boot without propolis-server panicking.