block: disk_file: Add PhysicalSize trait

Host allocation size for file-backed disk images. Reports
actual bytes occupied on the host filesystem. Not every
format supports this, e.g. network or memory backed disks.

Signed-off-by: Anatol Belski <anbelski@linux.microsoft.com>

block/src/disk_file.rs

@@ -42,3 +42,9 @@ pub trait DiskSize: Send + Debug {
     /// Virtual size of the disk image in bytes (reported capacity).
     fn logical_size(&self) -> BlockResult<u64>;
 }
+
+/// Host allocation size of a file-backed disk image.
+pub trait PhysicalSize: Send + Debug {
+    /// Actual bytes occupied on the host filesystem.
+    fn physical_size(&self) -> BlockResult<u64>;
+}