netmap: Drop netmap_can_send

This callback is called by main loop before polling s->fd, if it returns
false, the fd will not be polled in this iteration.

This is redundant with checks inside read callback. After this patch,
the data will be copied from s->fd to s->iov when it arrives. If the
device can't receive, it will be queued to incoming_queue, and when the
device status changes, this queue will be flushed.

Also remove the qemu_can_send_packet() check in netmap_send. If it's
true, we are good; if it's false, the qemu_sendv_packet_async would
return 0 and read poll will be disabled until netmap_send_completed is
called.

Signed-off-by: Fam Zheng <famz@redhat.com>
Message-id: 1433400324-7358-5-git-send-email-famz@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
