monitor: fix use after free

The function monitor_fdset_dup_fd_find_remove() references member of
'mon_fdset' which - when remove flag is set - may be freed in function
monitor_fdset_cleanup().
remove is set by monitor_fdset_dup_fd_remove which in practice
does not need the returned value, so make it void,
and return -1 from monitor_fdset_dup_fd_find_remove.

Reported-by: zhanghailiang <zhang.zhanghailiang@huawei.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>