docs: schema and docs for the midonet virtualport type

docs: schema and docs for the midonet virtualport type

Midonet is an opensource virtual networking that over lays the IP
network between hypervisors. Currently, such networks can be made
with the openvswitch virtualport type.

This patch, defines the schema and documentation that will serve
as basis for the follow up patches that will add support to libvirt
for using Midonet virtual ports for its interfaces. The schema
definition requires that the port profile expresses its interfaceid
as part of the port profile. For that reason, this is part of the
patch too.

Signed-off-by: Antoni Segura Puimedon <toni+libvirt@midokura.com>