rust/vmstate: Add unit test for pointer case

rust/vmstate: Add unit test for pointer case

Add a unit test to cover some patterns accepted by vmstate_of macro,
which correspond to the following C version macros:
 * VMSTATE_POINTER
 * VMSTATE_ARRAY_OF_POINTER

Note: Currently, vmstate_struct can't handle the pointer to structure
case. Leave this case as a FIXME and use vmstate_unused as a place
holder.

Signed-off-by: Zhao Liu <zhao1.liu@intel.com>
Link: https://lore.kernel.org/r/20250318130219.1799170-14-zhao1.liu@intel.com
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>