vmm: disk resize infrastructure

Add basic infrastructure so resize events are
propagated to the underlying disk implementation.

On-behalf-of: SAP thomas.prescher@sap.com
Signed-off-by: Thomas Prescher <thomas.prescher@cyberus-technology.de>

block/src/async_io.rs

@@ -18,6 +18,12 @@ pub enum DiskFileError {
     /// Failed creating a new AsyncIo.
     #[error("Failed creating a new AsyncIo")]
     NewAsyncIo(#[source] std::io::Error),
+    /// Unsupported operation.
+    #[error("Unsupported operation")]
+    Unsupported,
+    /// Resize failed
+    #[error("Resize failed")]
+    ResizeError(#[source] std::io::Error),
 }
 
 pub type DiskFileResult<T> = std::result::Result<T, DiskFileError>;
@@ -68,6 +74,10 @@ pub trait DiskFile: Send {
     fn topology(&mut self) -> DiskTopology {
         DiskTopology::default()
     }
+    fn resize(&mut self, _size: u64) -> DiskFileResult<()> {
+        Err(DiskFileError::Unsupported)
+    }
+
     /// Returns the file descriptor of the underlying disk image file.
     ///
     /// The file descriptor is supposed to be used for `fcntl()` calls but no