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

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 <zhao1.liu@intel.com>
Link: https://lore.kernel.org/r/20250318130219.1799170-4-zhao1.liu@intel.com
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>