rust/irq: Add a helper to convert [InterruptSource] to pointer

This is useful when taking an InterruptSource slice and passing it to C
function.

Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Zhao Liu <zhao1.liu@intel.com>
Link: https://lore.kernel.org/r/20250210030051.2562726-4-zhao1.liu@intel.com
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

rust/qemu-api/src/irq.rs

@@ -83,6 +83,12 @@ where
     pub(crate) const fn as_ptr(&self) -> *mut *mut IRQState {
         self.cell.as_ptr()
     }
+
+    #[allow(dead_code)]
+    pub(crate) const fn slice_as_ptr(slice: &[Self]) -> *mut *mut IRQState {
+        assert!(!slice.is_empty());
+        slice[0].as_ptr()
+    }
 }
 
 impl Default for InterruptSource {