instruction: 0.419
syscall: 0.296
runtime: 0.285



TCG Plugin exit callback not executing
Description of problem:
I cannot get the plugin exit callback to register/execute. I should see "Goodbye from plugin" but dont. I have also tried using `qemu_plugin_outs` without success.

**Update: If I make my test binary an infinite loop and kill it with CTRL-C, then the callback is called as expected. Am I just using it wrong?**
Steps to reproduce:
1. Configured QEMU with `--target-list=riscv32-linux-user,riscv64-linux-user --enable-plugins --disable-system`
2. Compiled plugin with 
```
gcc -I./qemu/include/qemu `pkg-config --libs glib-2.0` -O0 -fvisibility=hidden -Wall -shared -fPIC `pkg-config --cflags glib-2.0`
```
3. Compiled test binary (just a hello world) with `riscv64-unknown-elf-gcc test_qemu.c -o test_qemu`
4. Ran ./qemu/build/qemu-riscv64 -plugin ./test_plugin.so -d plugin ./test_qemu
Additional information:
test_plugin.c
```
#include <inttypes.h>
#include <assert.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <stdio.h>
#include <qemu-plugin.h>

QEMU_PLUGIN_EXPORT int qemu_plugin_version = QEMU_PLUGIN_VERSION;

static void vcpu_tb_trans(qemu_plugin_id_t id, struct qemu_plugin_tb *tb)
{
    int n_insns = qemu_plugin_tb_n_insns(tb);
    printf("> New TB of size %d\n", n_insns);

    for (int i = 0; i < n_insns; i++) {
        struct qemu_plugin_insn * insn = qemu_plugin_tb_get_insn(tb, i);
        char * disassembly = qemu_plugin_insn_disas(insn);
        printf(" > Instruciton: %s\n", disassembly);
    }
}

static void plugin_exit(qemu_plugin_id_t id, void *p)
{
    printf("> Goodbye from plugin. %d\n", id);
}

QEMU_PLUGIN_EXPORT int qemu_plugin_install(qemu_plugin_id_t id,
                                           const qemu_info_t *info,
                                           int argc, char **argv)
{
    printf("> Hello From Plugin!\n");
    qemu_plugin_register_vcpu_tb_trans_cb(id, vcpu_tb_trans);
    qemu_plugin_register_atexit_cb(id, plugin_exit, NULL);
    printf("> Everything was registered\n");
    return 0;
}
```