tap: Avoid extra iterations while closing file fd

Avoid iterations for fd 0, 1 & 2 when we are closing file fds in child process.

Signed-off-by: Pankaj Gupta <pagupta@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
pcnet: remove duplicate assignment

Signed-off-by: Prasad Joshi <prasadjoshi.linux@gmail.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>