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>
