char: do not use atexit cleanup handler

It turns out qemu is calling exit() in various places from various
threads without taking much care of resources state. The atexit()
cleanup handlers cannot easily destroy resources that are in use (by
the same thread or other).

Since c1111a24a3, TCG arm guests run into the following abort() when
running tests, the chardev mutex is locked during the write, so
qemu_mutex_destroy() returns an error:

 #0  0x00007fffdbb806f5 in raise () at /lib64/libc.so.6
 #1  0x00007fffdbb822fa in abort () at /lib64/libc.so.6
 #2  0x00005555557616fe in error_exit (err=<optimized out>, msg=msg@entry=0x555555c38c30 <__func__.14622> "qemu_mutex_destroy")
     at /home/drjones/code/qemu/util/qemu-thread-posix.c:39
 #3  0x0000555555b0be20 in qemu_mutex_destroy (mutex=mutex@entry=0x5555566aa0e0) at /home/drjones/code/qemu/util/qemu-thread-posix.c:57
 #4  0x00005555558aab00 in qemu_chr_free_common (chr=0x5555566aa0e0) at /home/drjones/code/qemu/qemu-char.c:4029
 #5  0x00005555558b05f9 in qemu_chr_delete (chr=<optimized out>) at /home/drjones/code/qemu/qemu-char.c:4038
 #6  0x00005555558b05f9 in qemu_chr_delete (chr=<optimized out>) at /home/drjones/code/qemu/qemu-char.c:4044
 #7  0x00005555558b062c in qemu_chr_cleanup () at /home/drjones/code/qemu/qemu-char.c:4557
 #8  0x00007fffdbb851e8 in __run_exit_handlers () at /lib64/libc.so.6
 #9  0x00007fffdbb85235 in  () at /lib64/libc.so.6
 #10 0x00005555558d1b39 in testdev_write (testdev=0x5555566aa0a0) at /home/drjones/code/qemu/backends/testdev.c:71
 #11 0x00005555558d1b39 in testdev_write (chr=<optimized out>, buf=0x7fffc343fd9a "", len=0) at /home/drjones/code/qemu/backends/testdev.c:95
 #12 0x00005555558adced in qemu_chr_fe_write (s=0x5555566aa0e0, buf=buf@entry=0x7fffc343fd98 "0q", len=len@entry=2) at /home/drjones/code/qemu/qemu-char.c:282

Instead of using a atexit() handler, only run the chardev cleanup as
initially proposed at the end of main(), where there are less chances
(hic) of conflicts or other races.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reported-by: Andrew Jones <drjones@redhat.com>
Message-Id: <20160704153823.16879-1-marcandre.lureau@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
