rust/vmstate: Fix num field when varray flags are set

Array type vmstate has the VMStateField with `num` equals its length.

When the varray vmstate is built based a array type, the `num` field
should be cleaned to 0, because varray uses `num_offset` instead of
`num` to store elements number information.

Signed-off-by: Zhao Liu <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Paolo Bonzini <[email protected]>

Author: trueptolemy

rust/qemu-api/src/vmstate.rs

@@ -275,6 +275,7 @@ impl VMStateField {
         assert!((self.flags.0 & VMStateFlags::VMS_ARRAY.0) != 0);
         self.flags = VMStateFlags(self.flags.0 & !VMStateFlags::VMS_ARRAY.0);
         self.flags = VMStateFlags(self.flags.0 | flag.0);
+        self.num = 0; // varray uses num_offset instead of num.
         self
     }