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

docs/Makefile.am

@@ -60,7 +60,12 @@ png = \
   libvirt-driver-arch.png \
   libvirt-object-model.png \
   madeWith.png \
-  et.png
+  et.png \
+  migration-managed-direct.png \
+  migration-managed-p2p.png \
+  migration-native.png \
+  migration-tunnel.png \
+  migration-unmanaged-direct.png
 
 gif = \
   architecture.gif \
@@ -85,7 +90,12 @@ fig = \
   libvirt-net-physical.fig \
   libvirt-daemon-arch.fig \
   libvirt-driver-arch.fig \
-  libvirt-object-model.fig
+  libvirt-object-model.fig \
+  migration-managed-direct.fig \
+  migration-managed-p2p.fig \
+  migration-native.fig \
+  migration-tunnel.fig \
+  migration-unmanaged-direct.fig
 
 EXTRA_DIST=					\
   apibuild.py \