block: Remove DiskBackend dispatch enum

All disk format backends now implement AsyncFullDiskFile directly.
The DiskBackend enum that dispatched between Legacy and Next arms
is no longer needed since the factory returns trait objects and vmm
no longer constructs format types manually.

Replace DiskBackend with Box<dyn AsyncFullDiskFile> in the Block
struct and its constructor. Remove the DiskBackend::Next wrapping
in device_manager and the fuzz target.

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

vmm/src/device_manager.rs

@@ -33,7 +33,6 @@ use arch::layout::{APIC_START, IOAPIC_SIZE, IOAPIC_START};
 use arch::{DeviceType, MmioDeviceInfo};
 use arch::{NumaNodes, layout};
 use block::ImageType;
-use block::disk_file::DiskBackend;
 use block::error::BlockError;
 use block::factory::{DiskOpenOptions, open_disk};
 #[cfg(target_arch = "riscv64")]
@@ -2719,8 +2718,6 @@ impl DeviceManager {
                 warn!("Enabling backing_files option only applies for QCOW2 files");
             }
 
-            let image = DiskBackend::Next(opened.disk);
-
             let rate_limit_group =
                 if let Some(rate_limiter_cfg) = disk_cfg.rate_limiter_config.as_ref() {
                     // Create an anonymous RateLimiterGroup that is dropped when the Disk
@@ -2764,7 +2761,7 @@ impl DeviceManager {
 
             let mut virtio_block = virtio_devices::Block::new(
                 id.clone(),
-                image,
+                opened.disk,
                 disk_cfg
                     .path
                     .as_ref()