Re: [libvirt] documenting the 802.1Qbg parameters of a 'direct' interface

This patchs adds documentation about the 802.1Qbg related parameters of the virtualport element in a 'direct' interface definition.

+ <p> + The network access of direct attached virtual machines can be + managed by the hardware switch to which the physical interface + of the host machine is connected to. + <p> + The interface can have additional parameters as shown below, + if the switch is conforming to the IEEE 802.1Qbg standard. + The parameters of the virtualport element are documented in more detail + in the IEEE 802.1Qbg standard. The values are network specific and + should be provided by the network administrator. In 802.1Qbg terms, + the Virtual Station Interface (VSI) represents the virtual inferface