rust: use MaybeUninit::zeroed() in const context

Reviewed-by: Manos Pitsidianakis <manos.pitsidianakis@linaro.org>
Reviewed-by: Zhao Liu <zhao1.liu@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

rust/hw/timer/hpet/src/fw_cfg.rs

@@ -4,7 +4,7 @@
 
 use std::ptr::addr_of_mut;
 
-use qemu_api::{cell::bql_locked, impl_zeroable, zeroable::Zeroable};
+use qemu_api::{cell::bql_locked, zeroable::Zeroable};
 
 /// Each `HPETState` represents a Event Timer Block. The v1 spec supports
 /// up to 8 blocks. QEMU only uses 1 block (in PC machine).
@@ -18,7 +18,7 @@ pub struct HPETFwEntry {
     pub min_tick: u16,
     pub page_prot: u8,
 }
-impl_zeroable!(HPETFwEntry);
+unsafe impl Zeroable for HPETFwEntry {}
 
 #[repr(C, packed)]
 #[derive(Copy, Clone, Default)]
@@ -26,7 +26,7 @@ pub struct HPETFwConfig {
     pub count: u8,
     pub hpet: [HPETFwEntry; HPET_MAX_NUM_EVENT_TIMER_BLOCK],
 }
-impl_zeroable!(HPETFwConfig);
+unsafe impl Zeroable for HPETFwConfig {}
 
 #[allow(non_upper_case_globals)]
 #[no_mangle]