migration: allow to prioritize save state entries

During migration, save state entries are saved/loaded without a specific
order - we just traverse the savevm_state.handlers list and do it one by
one. This might not be enough.

There are requirements that we need to load specific device's vmstate
first before others. For example, VT-d IOMMU contains DMA address
remapping information, which is required by all the PCI devices to do
address translations. We need to make sure IOMMU's device state is
loaded before the rest of the PCI devices, so that DMA address
translation can work properly.

This patch provide a VMStateDescription.priority value to allow specify
the priority of the saved states. The loadvm operation will be done with
those devices with higher vmsd priority.

Before this patch, we are possibly achieving the ordering requirement by
an assumption that the ordering will be the same with the ordering that
objects are created. A better way is to mark it out explicitly in the
VMStateDescription table, like what this patch does.

Current ordering logic is still naive and slow, but after all that's not
a critical path so IMO it's a workable solution for now.

Signed-off-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>