rust/vmstate: Re-implement VMState trait for timer binding

rust/vmstate: Re-implement VMState trait for timer binding

At present, Rust side has a timer binding "timer::Timer", so the vmstate
for timer should base on that binding instead of the raw
"binding::QEMUTimer".

It's possible to apply impl_vmstate_transparent for cell::Opaque and
then impl_vmstate_forward for timer::Timer. But binding::QEMUTimer
shouldn't be used directly, so that vmstate for such raw timer type is
useless.

Thus, apply impl_vmstate_scalar for timer::Timer. And since Opaque<> is
useful, apply impl_vmstate_transparent for cell::Opaque as well.

Signed-off-by: Zhao Liu <zhao1.liu@intel.com>
Link: https://lore.kernel.org/r/20250318130219.1799170-10-zhao1.liu@intel.com
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>