Add VMSTATE_STRUCT_VARRAY_KNOWN

Add VMSTATE_STRUCT_VARRAY_KNOWN

At the moment we have VMSTATE_STRUCT_ARRAY that requires
the field is declared as an array of fixed size.
We also have VMSTATE_STRUCT_VARRAY_UINT* that allows
a field declared as a pointer, but requires that the length
is a field member in the structure being loaded/saved.

VMSTATE_STRUCT_VARRAY_KNOWN is for arrays defined as pointers
yet we somehow know the length of.

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Amit Shah <amit.shah@redhat.com>