rust/vmstate: Relax array check when build varray in vmstate_struct

rust/vmstate: Relax array check when build varray in vmstate_struct

The varry of structure created by vmstate_struct is different with
vmstate_of. This is because vmstate_struct uses the `vmsd` to traverse
the vmstates of structure's fields, rather than treating the structure
directly as a well-defined vmstate.

Therefore, there's no need to check array flag when building varray by
vmstate_struct.

Signed-off-by: Zhao Liu <zhao1.liu@intel.com>
Link: https://lore.kernel.org/r/20250318130219.1799170-9-zhao1.liu@intel.com
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>