rust: make properties array immutable

rust: make properties array immutable

Now that device_class_set_props() takes a const pointer, the only part of
"define_property!" that needs to be non-const is the call to try_into().
This in turn will only break if offset_of returns a value with the most
significant bit set (i.e. a struct size that is >=2^31 or >= 2^63,
respectively on 32- and 64-bit system), which is impossible.

Just use a cast and clean everything up to remove the run-time
initialization.  This also removes a use of OnceLock, which was only
stabilized in 1.70.0.

Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>