rust: qdev: add clock creation

rust: qdev: add clock creation

Add a Rust version of qdev_init_clock_in, which can be used in
instance_init.  There are a couple differences with the C
version:

- in Rust the object keeps its own reference to the clock (in addition to
  the one embedded in the NamedClockList), and the reference is dropped
  automatically by instance_finalize(); this is encoded in the signature
  of DeviceClassMethods::init_clock_in, which makes the lifetime of the
  clock independent of that of the object it holds.  This goes unnoticed
  in the C version and is due to the existence of aliases.

- also, anything that happens during instance_init uses the pinned_init
  framework to operate on a partially initialized object, and is done
  through class methods (i.e. through DeviceClassMethods rather than
  DeviceMethods) because the device does not exist yet.  Therefore, Rust
  code *must* create clocks from instance_init, which is stricter than C.

Reviewed-by: Zhao Liu <zhao1.liu@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>