virtio-net: enable configurable tx queue size

This patch enables the virtio-net tx queue size to be configurable
between 256 (the default queue size) and 1024 by the user when the
vhost-user backend is used.

Currently, the maximum tx queue size for other backends is 512 due
to the following limitations:
- QEMU backend: the QEMU backend implementation in some cases may
send 1024+1 iovs to writev.
- Vhost_net backend: there are possibilities that the guest sends
a vring_desc of memory which crosses a MemoryRegion thereby
generating more than 1024 iovs after translation from guest-physical
address in the backend.

Signed-off-by: Wei Wang <wei.w.wang@intel.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
