target/i386: raise FERR interrupt with iothread locked

Otherwise tcg_handle_interrupt() triggers an assertion failure:

  #5  0x0000555555c97369 in tcg_handle_interrupt (cpu=0x555557434cb0, mask=2) at ../accel/tcg/tcg-accel-ops.c:83
  #6  tcg_handle_interrupt (cpu=0x555557434cb0, mask=2) at ../accel/tcg/tcg-accel-ops.c:81
  #7  0x0000555555b4d58b in pic_irq_request (opaque=<optimized out>, irq=<optimized out>, level=1) at ../hw/i386/x86.c:555
  #8  0x0000555555b4f218 in gsi_handler (opaque=0x5555579423d0, n=13, level=1) at ../hw/i386/x86.c:611
  #9  0x00007fffa42bde14 in code_gen_buffer ()
  #10 0x0000555555c724bb in cpu_tb_exec (cpu=cpu@entry=0x555557434cb0, itb=<optimized out>, tb_exit=tb_exit@entry=0x7fffe9bfd658) at ../accel/tcg/cpu-exec.c:457

Cc: qemu-stable@nongnu.org
Resolves: https://gitlab.com/qemu-project/qemu/-/issues/1808
Reported-by: NyanCatTW1 <https://gitlab.com/a0939712328

>
Co-developed-by: Richard Henderson <richard.henderson@linaro.org&gt;'>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>