vl: fix resource leak with monitor_fdset_add_fd

monitor_fdset_add_fd returns an AddfdInfo struct (used by the QMP
command add_fd).  Free it.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
