block: Add supports_zero_flag trait method

Add supports_zero_flag() to DiskFile trait to indicate whether a disk
format can mark clusters/blocks as reading zeros without deallocating
storage.

QCOW2 supports this via the zero flag in L2 entries. VHDX also has
PAYLOAD_BLOCK_ZERO state for this, though it's not yet implemented in
cloud-hypervisor.

This enables DISCARD to be advertised even with sparse=false for formats
with zero-flag support, since they can mark regions as zeros (keeps
storage allocated) instead of requiring full deallocation.

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

block/src/async_io.rs

@@ -84,6 +84,12 @@ pub trait DiskFile: Send {
         false
     }
 
+    /// Indicates support for zero flag optimization in WRITE_ZEROES. Override
+    /// to return true when supported.
+    fn supports_zero_flag(&self) -> bool {
+        false
+    }
+
     /// Returns the file descriptor of the underlying disk image file.
     ///
     /// The file descriptor is supposed to be used for `fcntl()` calls but no

block/src/qcow_sync.rs

@@ -80,6 +80,10 @@ impl DiskFile for QcowDiskSync {
         true
     }
 
+    fn supports_zero_flag(&self) -> bool {
+        true
+    }
+
     fn fd(&mut self) -> BorrowedDiskFd<'_> {
         BorrowedDiskFd::new(self.qcow_file.lock().unwrap().as_raw_fd())
     }