fix: iommu InterruptRemapTable — add DmaBuffer-backed allocation, fix compile

interrupt.rs:
- InterruptRemapTable now owns optional DmaBuffer for self-allocated tables
- new_allocated(entry_count) constructor allocates physically contiguous
  DMA memory via DmaBuffer::allocate, returns Result
- new(base_addr, size) still works for externally-provided tables
- private addr() helper replaces direct 'base' field access
- len_encoding() returns AMD-Vi log2-encoded IRT length for DTE entries
- physical_address() returns table base physical address
- Remove unused 'warn' and 'error' imports from log crate

amd_vi.rs:
- Use InterruptRemapTable::new_allocated instead of ::new for IRT init
- Cast len_encoding() from u64 to u8 for DeviceTableEntry::set_int_table_len

Verified: iommu crate compiles clean (0 errors, 0 warnings).

local/recipes/system/iommu/source/src/amd_vi.rs

@@ -99,7 +99,7 @@ impl AmdViUnit {
             CommandBuffer::new(DEFAULT_CMD_ENTRIES).map_err(|err| err.to_string())?;
         let event_log = EventLog::new(DEFAULT_EVT_ENTRIES).map_err(|err| err.to_string())?;
         let interrupt_table =
-            InterruptRemapTable::new(DEFAULT_IRT_ENTRIES).map_err(|err| err.to_string())?;
+            InterruptRemapTable::new_allocated(DEFAULT_IRT_ENTRIES).map_err(|err| err.to_string())?;
 
         self.program_bars(&device_table, &command_buffer, &event_log)?;
         self.reset_ring_pointers()?;
@@ -155,7 +155,7 @@ impl AmdViUnit {
         entry.set_interrupt_remap(true);
         entry.set_interrupt_write(true);
         entry.set_interrupt_control(0x02);
-        entry.set_int_table_len(interrupt_table.len_encoding());
+        entry.set_int_table_len(interrupt_table.len_encoding() as u8);
         entry.set_int_remap_table_ptr(interrupt_table.physical_address() as u64);
 
         device_table.set_entry(bdf.raw(), &entry);