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: NDr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: NMichael S. Tsirkin <mst@redhat.com>
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>
Reviewed-by: NAmit Shah <amit.shah@redhat.com>

include/migration/vmstate.h

@@ -374,6 +374,19 @@ extern const VMStateInfo vmstate_info_bitmap;
     .offset       = vmstate_offset_array(_state, _field, _type, _num),\
 }
 
+/* a variable length array (i.e. _type *_field) but we know the
+ * length
+ */
+#define VMSTATE_STRUCT_VARRAY_KNOWN(_field, _state, _num, _version, _vmsd, _type) { \
+    .name       = (stringify(_field)),                               \
+    .num          = (_num),                                          \
+    .version_id = (_version),                                        \
+    .vmsd       = &(_vmsd),                                          \
+    .size       = sizeof(_type),                                     \
+    .flags      = VMS_STRUCT|VMS_ARRAY,                              \
+    .offset     = offsetof(_state, _field),                          \
+}
+
 #define VMSTATE_STRUCT_VARRAY_UINT8(_field, _state, _field_num, _version, _vmsd, _type) { \
     .name       = (stringify(_field)),                               \
     .num_offset = vmstate_offset_value(_state, _field_num, uint8_t), \