iommufd-ioctls: Add IommuFd::new_from_fd

Accept an externally-opened /dev/iommu file to create an IommuFD
instance.

Signed-off-by: Bo Chen <bchen@crusoe.ai>

Author: likebreath

iommufd-ioctls/CHANGELOG.md

@@ -5,6 +5,8 @@
 ## Added
 - [[9]](https://github.com/cloud-hypervisor/iommufd/pull/9) Add `Iommufd::destroy_iommu_object` to release iommufd
   objects explicitly.
+- [[9]](https://github.com/cloud-hypervisor/iommufd/pull/9) Add `Iommufd::new_from_fd` to construct a `Iommufd` from
+  a pre-opened iommufd file.
 
 ## Fixed
 

iommufd-ioctls/src/iommufd_ioctls.rs

@@ -26,6 +26,12 @@ impl IommuFd {
         Ok(IommuFd { iommufd })
     }
 
+    /// New from an already-opened `/dev/iommu` file. Caller transfers
+    /// ownership; closing the fd is now `IommuFd`'s responsibility.
+    pub fn new_from_fd(file: File) -> Self {
+        IommuFd { iommufd: file }
+    }
+
     pub fn destroy_iommu_object(&self, id: u32) -> Result<()> {
         let destroy_data = iommu_destroy {
             size: std::mem::size_of::<iommu_destroy>() as u32,