#41 Stronger cheking of attributes in <net>

[Fermín Galán]

Currently there are some attributes that only are applied to either mode="virtual_bridge" or mode="uml_switch" in <net> tag. In particular:

- virtual_bridge attributes: external, vlan
- uml_switch attributes: hub, sock, uml_switch_binary, capture_file, capture_expression, capture_dev

It should be checked that attributes each class are only used in the proper mode. For example, a <net mode="virtual_bridge" uml_switch_binary="/some/file" ...> will cause an error during semantic chekcing. Note that the current version (<=1.8.0) ignore attributes in that case.