stash away SCM_RIGHTS fd until a getfd command arrives

If there is already a fd in s->msgfd before recvmsg it is
closed by parts that this patch does not touch.  So, only
one descriptor can be "leaked" by attaching it to a command
other than getfd.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>

monitor.c

@@ -2415,15 +2415,6 @@ static int do_getfd(Monitor *mon, const QDict *qdict, QObject **ret_data)
         return -1;
     }
 
-    fd = dup(fd);
-    if (fd == -1) {
-        if (errno == EMFILE)
-            qerror_report(QERR_TOO_MANY_FILES);
-        else
-            qerror_report(QERR_UNDEFINED_ERROR);
-        return -1;
-    }
-
     QLIST_FOREACH(monfd, &mon->fds, next) {
         if (strcmp(monfd->name, fdname) != 0) {
             continue;

qemu-char.c

@@ -2000,8 +2000,9 @@ static void tcp_chr_process_IAC_bytes(CharDriverState *chr,
 static int tcp_get_msgfd(CharDriverState *chr)
 {
     TCPCharDriver *s = chr->opaque;
-
-    return s->msgfd;
+    int fd = s->msgfd;
+    s->msgfd = -1;
+    return fd;
 }
 
 #ifndef _WIN32
@@ -2089,10 +2090,6 @@ static void tcp_chr_read(void *opaque)
             tcp_chr_process_IAC_bytes(chr, s, buf, &size);
         if (size > 0)
             qemu_chr_read(chr, buf, size);
-        if (s->msgfd != -1) {
-            close(s->msgfd);
-            s->msgfd = -1;
-        }
     }
 }