rust/hpet: Support migration

Based on commit 1433e38cc8 ("hpet: do not overwrite properties on
post_load"), add the basic migration support to Rust HPET.

The current migration implementation introduces multiple unsafe
callbacks. Before the vmstate builder, one possible cleanup approach is
to wrap callbacks in the vmstate binding using a method similar to the
vmstate_exist_fn macro.

However, this approach would also create a lot of repetitive code (since
vmstate has so many callbacks: pre_load, post_load, pre_save, post_save,
needed and dev_unplug_pending). Although it would be cleaner, it would
somewhat deviate from the path of the vmstate builder.

Therefore, firstly focus on completing the functionality of HPET, and
those current unsafe callbacks can at least clearly indicate the needed
functionality of vmstate. The next step is to consider refactoring
vmstate to move towards the vmstate builder direction.

Additionally, update rust.rst about Rust HPET can support migration.

Signed-off-by: Zhao Liu <zhao1.liu@intel.com>
Link: https://lore.kernel.org/r/20250414144943.1112885-9-zhao1.liu@intel.com
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

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