migration: Add VMSTATE_WITH_TMP

VMSTATE_WITH_TMP is for handling structures where some calculation
or rearrangement of the data needs to be performed before the data
hits the wire.
For example,  where the value on the wire is an offset from a
non-migrated base, but the data in the structure is the actual pointer.

To use it, a temporary type is created and a vmsd used on that type.
The first element of the type must be 'parent' a pointer back to the
type of the main structure.  VMSTATE_WITH_TMP takes care of allocating
and freeing the temporary before running the child vmsd.

The post_load/pre_save on the child vmsd can copy things from the parent
to the temporary using the parent pointer and do any other calculations
needed; it can then use normal VMSD entries to do the actual data
storage without having to fiddle around with qemu_get_*/qemu_put_*

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Message-Id: <20170203160651.19917-3-dgilbert@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>

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