monitor: Fix order in monitor_cleanup()

We can only destroy Monitor objects after we're sure that they are not
in use by the dispatcher coroutine any more. This fixes crashes like the
following where we tried to destroy a monitor mutex while the dispatcher
coroutine still holds it:

 (gdb) bt
 #0  0x00007fe541cf4bc5 in raise () at /lib64/libc.so.6
 #1  0x00007fe541cdd8a4 in abort () at /lib64/libc.so.6
 #2  0x000055c24e965327 in error_exit (err=16, msg=0x55c24eead3a0 <__func__.33> "qemu_mutex_destroy") at ../util/qemu-thread-posix.c:37
 #3  0x000055c24e9654c3 in qemu_mutex_destroy (mutex=0x55c25133e0f0) at ../util/qemu-thread-posix.c:70
 #4  0x000055c24e7cfaf1 in monitor_data_destroy_qmp (mon=0x55c25133dfd0) at ../monitor/qmp.c:439
 #5  0x000055c24e7d23bc in monitor_data_destroy (mon=0x55c25133dfd0) at ../monitor/monitor.c:615
 #6  0x000055c24e7d253a in monitor_cleanup () at ../monitor/monitor.c:644
 #7  0x000055c24e6cb002 in qemu_cleanup () at ../softmmu/vl.c:4549
 #8  0x000055c24e0d259b in main (argc=24, argv=0x7ffff66b0d58, envp=0x7ffff66b0e20) at ../softmmu/main.c:51

Reported-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Message-Id: <20201013125027.41003-1-kwolf@redhat.com>
Tested-by: Ben Widawsky <ben.widawsky@intel.com>
Tested-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>