ui: migrate: add source node field and rebalance field positions

ui: migrate: add source node field and rebalance field positions

This makes the window more symmetric, and additional has the
following small advantages:
* to the left we now have static fields only, user modifiable ones
  are all to the right (with shorter distance to the migrate "submit"
  button here)
* if one starts the migration from the tree's context menu it may not
  be really clear where the VM currently is located at, so showing
  the source node can help (especially on bigger clusters with a
  huge target node list)
* more symmetric

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>