Docs: Remove zero-copy QEMU limitation.

Docs: Remove zero-copy QEMU limitation.

Remove note regarding zero-copy compatibility with QEMU >= 2.7.

When zero-copy was introduced to OVS it was incompatible with QEMU >=
2.7. This issue has since been fixed in DPDK with commit
803aeecef123 ("vhost: fix dequeue zero copy with virtio1") and
backported to DPDK LTS branches. Remove the reference to this
issue in the zero-copy documentation.

Cc: Ciara Loftus <ciara.loftus@intel.com>
Acked-by: Ciara Loftus <ciara.loftus@intel.com>
Signed-off-by: Ian Stokes <ian.stokes@intel.com>