No network resource in context if not specified in its JSON configuration

[noxdafox]

The user might provide his/her own libvirt network and attach the domain to it via its XML configuration.

In such cases, the Context object will have the network resource still set to None. This is due to the fact that the Context Factory is not aware of XML specific configurations.

This lead to bugs in the network based Plugins as they assume the presence of a libvirt network but they cannot reach it.

Possible solutions are:

• If no network is specified in the Context's JSON configuration, parse the XML and try to understand if the domain is attached to any network. This is a bit controversial as we try to be as much "libvirt config agnostic" as possible.
• Once the domain is created, try to lookup to which network is attached. This solution is simpler but requires libvirt APIs support. Investigation required.

[noxdafox]

Try out combining these two:

http://libvirt.org/html/libvirt-libvirt-domain.html#virDomainInterfaceAddresses
http://libvirt.org/html/libvirt-libvirt-network.html#virNetworkLookupByName

EDIT: it turns out this solution does not work.

Proposed one parses the XML in order to guess the network name and then looks it up via the hypervisor's connection.

http://libvirt.org/html/libvirt-libvirt-domain.html#virDomainInterfaceAddresses could enhance Context's IP address retrieval though.

[noxdafox]

Fix merged.