slirp: use exit notifier for slirp_smb_cleanup

We would like to move back net_cleanup() at the end of main function,
like it used to be until f30dbae6, but minimum
cleanup is needed regardless at exit() time for slirp's SMB
functionality.  Use an exit notifier to call slirp_smb_cleanup.
If net_cleanup() is called first, then remove the exit notifier as it
will become a dangling pointer otherwise.
Reviewed-by: NMarc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: NJason Wang <jasowang@redhat.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

net/slirp.c

@@ -38,6 +38,7 @@
 #include "slirp/libslirp.h"
 #include "slirp/ip6.h"
 #include "sysemu/char.h"
+#include "sysemu/sysemu.h"
 #include "qemu/cutils.h"
 
 static int get_str_sep(char *buf, int buf_size, const char **pp, int sep)
@@ -76,6 +77,7 @@ typedef struct SlirpState {
     NetClientState nc;
     QTAILQ_ENTRY(SlirpState) entry;
     Slirp *slirp;
+    Notifier exit_notifier;
 #ifndef _WIN32
     char smb_dir[128];
 #endif
@@ -118,11 +120,18 @@ static ssize_t net_slirp_receive(NetClientState *nc, const uint8_t *buf, size_t
     return size;
 }
 
+static void slirp_smb_exit(Notifier *n, void *data)
+{
+    SlirpState *s = container_of(n, SlirpState, exit_notifier);
+    slirp_smb_cleanup(s);
+}
+
 static void net_slirp_cleanup(NetClientState *nc)
 {
     SlirpState *s = DO_UPCAST(SlirpState, nc, nc);
 
     slirp_cleanup(s->slirp);
+    qemu_remove_exit_notifier(&s->exit_notifier);
     slirp_smb_cleanup(s);
     QTAILQ_REMOVE(&slirp_stacks, s, entry);
 }
@@ -349,6 +358,8 @@ static int net_slirp_init(NetClientState *peer, const char *model,
     }
 #endif
 
+    s->exit_notifier.notify = slirp_smb_exit;
+    qemu_add_exit_notifier(&s->exit_notifier);
     return 0;
 
 error: