The sequence of instructions exposes an issue:
  sti
  hlt

Interrupts cannot be delivered to hvf after hlt instruction cpu because
HF_INHIBIT_IRQ_MASK is set just before hlt is handled and never reset
after moving instruction pointer beyond hlt.

So, after hvf_vcpu_exec() returns, CPU thread gets locked up forever in
qemu_wait_io_event() (cpu_thread_is_idle() evaluates inhibition
flag and considers the CPU idle if the flag is set).

Cc: Cameron Esfahani <dirty@apple.com>
Signed-off-by: Roman Bolshakov <r.bolshakov@yadro.com>
Message-Id: <20200328174411.51491-1-r.bolshakov@yadro.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>