block: vhd: Remove catch all in physical_size boundary

FixedVhd::physical_size() can only return Error::GetFileMetadata.
Replace the catch-all arm with unreachable!() so future error
variants are not silently mapped to a generic Io classification.

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

block/src/fixed_vhd_async.rs

@@ -36,7 +36,7 @@ impl disk_file::PhysicalSize for FixedVhdDiskAsync {
             crate::Error::GetFileMetadata(io) => {
                 BlockError::new(BlockErrorKind::Io, crate::Error::GetFileMetadata(io))
             }
-            _ => BlockError::new(BlockErrorKind::Io, e),
+            _ => unreachable!("unexpected error from FixedVhd::physical_size(): {e}"),
         })
     }
 }

block/src/fixed_vhd_sync.rs

@@ -36,7 +36,7 @@ impl disk_file::PhysicalSize for FixedVhdDiskSync {
             crate::Error::GetFileMetadata(io) => {
                 BlockError::new(BlockErrorKind::Io, crate::Error::GetFileMetadata(io))
             }
-            _ => BlockError::new(BlockErrorKind::Io, e),
+            _ => unreachable!("unexpected error from FixedVhd::physical_size(): {e}"),
         })
     }
 }