rust: qemu_api: introduce MaybeUninit field projection

rust: qemu_api: introduce MaybeUninit field projection

Add a macro that makes it possible to convert a MaybeUninit<> into
another MaybeUninit<> for a single field within it.  Furthermore, it is
possible to use the resulting MaybeUninitField<> in APIs that take the
parent object, such as memory_region_init_io().

This allows removing some of the undefined behavior from instance_init()
functions, though this may not be the definitive implementation.

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