Add documentation about migration.

Add documentation about migration.

This adds a page documenting many aspects of migration:

 - The types of migration (managed direct, p2p, unmanaged direct)
 - Data transports (native, tunnelled)
 - Migration URIs
 - Config file handling
 - Example scenarios

* libvirt.css: Rules for data tables and diagrams
* Makefile.am: Include extra png/fig files
* migration-managed-direct.fig, migration-managed-direct.png,
  migration-managed-direct.png, migration-managed-p2p.png,
  migration-native.fig, migration-native.png,
  migration-tunnel.fig, migration-tunnel.png,
  migration-unmanaged-direct.fig, migration-unmanaged-direct.png:
  Diagrams of migration
* migration.html.in, sitemap.html.in: New migration doc