doc: update networking with Qemu section

Copy the documentation from the Wiki over here, the cross linking
between online documentation and Wiki and duplication of information was
inconsistent and confusing. Put everything in one place.

Change-Id: Ia3aaec1b431477e2ec54dbec2ccaa655870b0ee3
Signed-off-by: Anas Nashif <anas.nashif@intel.com>