block: Add sparse operations capability query

Add capability query to DiskFile trait to check backend
support for sparse operations (punch hole, write zeroes,
discard). Only advertise VIRTIO_BLK_F_DISCARD and
VIRTIO_BLK_F_WRITE_ZEROES when the backend supports these
operations.

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

block/src/raw_async.rs

@@ -13,7 +13,7 @@ use vmm_sys_util::eventfd::EventFd;
 use crate::async_io::{
     AsyncIo, AsyncIoError, AsyncIoResult, BorrowedDiskFd, DiskFile, DiskFileError, DiskFileResult,
 };
-use crate::{BatchRequest, DiskTopology, RequestType};
+use crate::{BatchRequest, DiskTopology, RequestType, probe_sparse_support};
 
 pub struct RawFileDisk {
     file: File,
@@ -59,6 +59,10 @@ impl DiskFile for RawFileDisk {
         self.file.set_len(size).map_err(DiskFileError::ResizeError)
     }
 
+    fn supports_sparse_operations(&self) -> bool {
+        probe_sparse_support(&self.file)
+    }
+
     fn fd(&mut self) -> BorrowedDiskFd<'_> {
         BorrowedDiskFd::new(self.file.as_raw_fd())
     }