Add worker->loadvm_commands. qemu will uses this to send a series of
commands needed to restore state after savevm/loadvm and migration.
That will be one create-surface command per surface and one cursor-set
command for the local pointer.
The worker->save/load functions are not needed any more.
Likewise the interface->{get,set}_save_data callbacks.
Surfaces created via loadvm_commands *will* not be cleared. Also
primary surfaces are not cleared any more (unconditionally, although
we could do that conditionally on loadvm using the flags field in
QXLSurfaceCreate).
