This patch adds the missing NetClient->receive_disabled logic in the
sendv delivery code path.  It seems that commit
893379ef

 ("net: disable receiving if
client returns zero") only added the logic to qemu_deliver_packet() and
not qemu_deliver_packet_iov().

The receive_disabled flag should be automatically set when .receive(),
.receive_raw(), or .receive_iov() return 0.  No further packets will be
delivered to the NetClient until the receive_disabled flag is cleared
again by calling qemu_flush_queued_packets().

Typically the NetClient will wait until its file descriptor becomes
writable and then invoke qemu_flush_queued_packets() to resume
transmission.

Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>